Very few Dark Lords remained, and only 50 joined them:

Everywhere he looked he saw the minions of the Sith: battle ragers, assassins, and apprentices. But there were precious few Sith Masters. The seemingly endless war against the Jedi on the battlefields of Ruusan was taking a heavy toll on Kaan's Brotherhood of Darkness. Without reinforcements they would be forced to retreat-or be wiped out by General Hoth and his hated Army of Light.

---

The five Interlopers-small, midrange multitroop transport ships used by the Sith-swept in low over Ruusan's horizon. Each vessel carried a crew of ten, comprised entirely of former students and Masters from Korriban's Academy.
...
A small collection of Sith Masters, Kaan standing at their head, waited to greet the reinforcements as they disembarked. They might have been only fifty in number, but each one of them was a Sith Lord: more powerful than an entire division of soldiers.

The Brotherhood was at full power with the addition of 50 Sith:

He lifted his voice so that it carried throughout the camp, his words booming across the tented landscape. "Now that you are here, the Brotherhood of Darkness is at last whole!"

The troops in camp paused and looked up at him. A few rose expectantly to their feet. There was fire in that single bold statement; it rekindled hope from the damp ashes of their fatigue and despair.

"The full power of the Sith Lords is now united here on Ruusan," he continued, projecting his words to even the most distant of his followers. Reaching out to them with the undeniable power of the Force, he fed them, rejuvenated them, and filled their hollow spirits. "We are strong. Stronger than the Jedi. We are the champions of the dark side, and we will crush Lord Hoth and his servants of light!"


A few died before the ritual commenced, including Kas'im, who was more powerful than any single Dark Lord besides maybe Kaan:

Most of the dead were likely to be followers of the Jedi or minions of the Sith, rather than actual Jedi Knights or members of the Brotherhood, though he noticed dark Sith robes on a handful of the bodies.

He wore the robes of the Sith, and the shattered remains of a lightsaber's hilt lay near his outstretched hand. Bane recognized him as one of the lesser students from the Academy on Korriban: so weak in the dark side, it wasn't even worth the bother of learning his name. Yet he knew Bane.

She laughed. "If he could handle Kas'im, then I doubt anyone else will stand a chance against him. Anyone but me."

They had arrived in single vehicles and parked them on a cliff:

Kaan, Githany, and the rest of the Dark Lords had gathered atop a barren plateau overlooking the vast forests where Hoth and his army were hiding. They had come on their fliers: short-range, single-person, airborne vehicles front-mounted with heavy blaster guns. The fliers were parked at the edge of the plateau, fifty meters away from where the Sith sat in a loose circle. The ritual had begun.

The 50 who joined them were students and teachers, and the state of the army:

They have no morale left to destroy," Kopecz shot back, though he did lower his volume. "We can't defeat Jedi with ordinary soldiers. There are too many of them and not enough of us."
"By us you mean those worthy of joining the ranks of the Dark Lords," Kaan replied. He sighed and stared down at the holomap spread out on the table before him.
"You know what you have to de Kopecz told him, his voice losing some of the anger. He had chosen to follow Kaan; he wouldn't abandon him now. But he wasn't about to sit idly by and face certain defeat. "We face an army of Jedi Knights and Masters. We can't stand against them without our own Masters from the Academy. The students, too. All of them."
"They are mere apprentices," Kaan protested.
"They are the strongest of our order," Kopecz reminded him. "We both know even the lowliest students on Korriban are stronger than half the so-called Dark Lords here on Ruusan."
"Qordis's work is not yet complete. The students there still have so much to learn," Kaan insisted, though without much force. "So much untapped potential. The Academy represents the future of the Sith."
"If we cannot defeat the Jedi here on Ruusan, then we have no future!" Kopecz insisted.
Lord Kaan clutched his head with his hands, as if a great pain threatened to burst his skull in two. He began to tremble in the grip of some terrible palsy. Kopecz involuntarily stepped back.
It only took a few seconds for Kaan to regain his composure and lower his hands. The haunted look in his eyes was gone, replaced by the calm self-assurance that had drawn so many to the Brotherhood in the first place.
"You're right, old friend," he said. The words were smooth and easy; he spoke as if a great weight had been lifted from him. He radiated confidence and strength. He seemed to glow with a violet aura, as if he were the very embodiment of the dark side. And suddenly, inexplicably, Kopecz was reassured.
"I will send word to Qordis," Kaan continued, the Force emanating from him in palpable waves. "You are right. It is time for those at the Academy on Korriban to truly join the ranks of the Sith."

...

A small collection of Sith Masters, Kaan standing at their head, waited to greet the reinforcements as they disembarked. They might have been only fifty in number, but each one of them was a Sith Lord: more powerful than an entire division of soldiers.
As she made her way down the ship's exit ramp, Githany was quick to understand why their presence had been so urgently requested. Beyond the assemblage of Dark Lords the rest of the camp spread out to the limits of her vision, and all she could see was a picture of grim despair. Ragged, ramshackle tents arranged in tight rings of five housed the bulk of the army: cloth domiciles stained and torn by wind and rain. Scattered among them were repulsorcraft, heavy turrets, and other instruments of war. The equipment was caked with dried mud and spots of rust, as if efforts to keep it properly maintained had been abandoned.
The troops were spread out in small pockets, huddled around cook fires built in the circles of tents. Their uniforms were covered in dust and grime; many wore dirty bandages over wounds they had given up all hope of keeping clean or sterile. Their faces were all scarred by the bitter taste of far too many defeats at the hands of their enemy, and it was the hopelessness of their expressions that made the greatest impression.
Lord Qordis seemed similarly taken aback at the dismal scene, and he grimaced as Lord Kaan approached.
Kaan appeared thin, his face drawn and etched with lines of worry. His hair was bedraggled and unkempt. A day's worth of stubble shadowed his chin, making him look old and weary. He seemed physically smaller than Githany remembered him. Diminished. Less commanding. The spark she had found so compelling when she'd first met him was no longer there. His eyes had once burned with the fire of a man absolutely confident of his imminent success. Now they burned with something else. Desperation. Madness, perhaps. She couldn't help but wonder if Bane had been right.
"Welcome, Lord Qordis:" Kaan said, grasping the newcomer's arm in greeting. He released his grip and turned to address the rest of them. "Welcome, all of you, to Ruusan."
"I didn't expect to see your army in such sorry shape:' Qordis mumbled.
A look that might have been anger flickered across Kaan's features. Then it was gone, replaced by the beaming confidence Githany remembered. He threw his shoulders back and stood a little straighter.
"You can't judge the victor of a war without seeing the condition of both sides:' he said crisply. "The Jedi are in far worse shape. My intelligence reports that their casualties are far greater than ours. Their supplies are running low; their numbers are dwindling. We have medpacs, food, and greater numbers. And they do not have fresh reinforcements."
He lifted his voice so that it carried throughout the camp, his words booming across the tented landscape. "Now that you are here, the Brotherhood of Darkness is at last whole!"
The troops in camp paused and looked up at him. A few rose expectantly to their feet. There was fire in that single bold statement; it rekindled hope from the damp ashes of their fatigue and despair.
"The full power of the Sith Lords is now united here on Ruusan," he continued, projecting his words to even the most distant of his followers. Reaching out to them with the undeniable power of the Force, he fed them, rejuvenated them, and filled their hollow spirits. "We are strong. Stronger than the Jedi. We are the champions of the dark side, and we will crush Lord Hoth and his servants of light!"

- Darth Bane: Path of Destruction