Hundreds of Jedi reinforcements would have been enough for the Army Of Light to beat Kaan's Army (Pre Firestorm):

"But victory here is not assured. We may have Hoth's army surrounded and pinned down, but there's a Jedi fleet with hundreds of reinforcements lurking on the edges of this system."

"Their reinforcements are on the edge of the system," Kaan admitted with a nod, not bothering to deny what every single one of them knew to be fact. "Just as they have been for the past week. And that's exactly where they will stay: far away from the surface where they are needed.

"The bulk of our fleet is in orbit around Ruusan itself, and the Jedi lack the numbers or the firepower to break through our blockade. If they can't unite their numbers with those here on the surface, Hoth and his followers will fall. And once we have finished them off we can mop up the tattered remnants of the Order at our leisure."

The Sith troops outside the caves of the Thought Bomb outnumbered the Jedi 3:1 BEFORE Hoth ONLY decided to take 100 Jedi into the Caves:

The cavern, though large, was crowded with the rest of the Brotherhood. Every surviving Sith Lord-with the notable exception of Darth Bane-was gathered with him here to make their final stand. The rest of his army was guarding the main entrances to the subterranean tunnels, with orders to hold off the inevitable Jedi attack for as long as possible.
Eventually those outside would be overwhelmed, but Kaan was confident their numbers would delay Hoth long enough for the ritual of the thought bomb to be completed.


The Sith soldiers guarding the entrance to the subterranean tunnels were strong in number but weak in spirit. They offered only token resistance to Farfalla and the rest of the Jedi advance units who came against them. The last battle of Ruusan quickly transformed into a mass surrender, with the enemy throwing down their weapons and begging for their lives.
Farfalla walked among his troops, surveying the scene. General Hoth was close behind with the bulk of the army. He'd be surprised to find the war already over when he arrived.
"How goes it?" Farfalla asked one of the unit commanders.
"The Sith troops have us outnumbered three to one," the commander answered gruffly. "And they're all trying to surrender at the same time. This is going to take awhile."

Kopecz told Farfalla Lord Kaan's plan about the Thought Bomb being created by the Dark Lords:

The Twi'lek's meaty hand lashed out to slap him away. "I chose my side long ago," he spat. "Promise me death, Jedi, and I will give you a warning. I will tell you Kaan's plan."
One look at the Dark Lord's wounds told Farfalla his enemy didn't have long to live in any case. "What do you know?"
Kopecz coughed, choking on the blood welling up in his throat. "Promise me, first," he wheezed.
"I will grant you death, if that is what you truly seek. I swear it."
The Twi'lek laughed, pink froth bubbling up on his lips. "Good. Death is an old friend. What Kaan has planned is far worse." And he told Farfalla about the thought bomb, his words sending a chill down the Jedi Master's spine. When Kopecz had finished he bowed his head and took a deep breath to gather his strength, then activated his lightsaber.

Farfalla then tells Lord Hoth to take 100 Jedi so Kaan fighting back would be hopeless:

"Surely you're not going in alone?" he (Farfalla) protested.
"I wish I could," Hoth replied. "But if I do the Dark Lords will simply take me down with their lightsabers. That would solve nothing. Kaan has to see that his only choice is to surrender or ..." He left the thought unspoken.
"You'll need enough Jedi to convince the Brotherhood that a physical battle would be hopeless. At least a hundred. Any less and he won't detonate the thought bomb." (Farfalla)
He (Hoth) selected exactly ninety-nine others to go with him. The decision was agonizingly difficult. If he took less, the Sith might be able to fight their way out of the cave and escape, only to detonate their thought bomb somewhere else. But the more he took, the more Jedi lives he might be needlessly throwing away.

100 Jedi were noted to slowly fill up the cavern:

As his eyes grew accustomed to the heavy shadows he was able to pick out the shapes of the Sith Lords, standing in a circle on the far side of the cave. They faced inward, their hands raised to its center. They stood without moving, their mouths hanging open, their features slack, their eyes blank. Cautiously, he approached the still forms, wondering if they were alive, dead, or trapped in some nightmare state in between.

Drawing closer he could make out a single figure standing in the center of the circle: Lord Kaan. He hadn't seen him at first; the middle of the ring was darker than the rest of the cave. There seemed to be a black cloud hovering above him, tendrils of inky darkness extending down to wrap and twist around him in a sinister embrace.

One look at the leader of the Brotherhood and any hope the general had of convincing Lord Kaan to listen to reason died. The Sith Lord's face was pale and taut; his features were stretched as if his skin had become too tight for his skull. A thin layer of ice coated his hair and lashes. His expression was one of cruel arrogance, and his left eye trembled and twitched uncontrollably. He stared straight ahead with a frozen intensity, unblinking and unmoving as Hoth and his Jedi slowly filled the cavern.

Kaan considered non-force sensitives a great addition to the Brotherhood's army:

Kopecz spit on the ground as he surveyed the camp. He was surrounded by an army, but it was an army of inferiors. 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 heavyset Twi'lek rose to his feet, spurred to action by the realization that something had to be done. He made his way through scattered pockets of soldiers, noticing how many were injured, exhausted, or simply defeated. By the time he reached the entrance to Lord Kaan's tent the contempt he felt for his so-called Brothers had reached a boiling point.
When Kopecz entered, Lord Kaan took one look at him and dismissed his other advisers with a sharp wave of his hand. They filed out, none of them daring to come too close.
"What is it, my old friend?" Kaan asked. His voice was charming as ever, but his eyes were wide and wild, like a hunted beast.
"Have you seen what passes for our army out there?" Kopecz snarled, poking a thumb over his shoulder as he walked slowly forward. "If this is all we have to stand against Lord Hoth, we may as was well burn our black robes and start practicing the Jedi Code."
"We have reinforcements coming," Lord Kaan assured him. "Two more full divisions of foot soldiers, another core of snipers. Half a platoon of repulsorcraft armed with heavy guns. There are many who are drawn to the glory of our cause. More and more each day. The Brotherhood of Darkness cannot fail."

Bane only considered there about 100 Dark Lords with the addition of the entire Academy (50) and assuming how many were still alive in the war:

The news that Bane had returned did not sit well with Qordis. The timing couldn't have been worse. Lord Kaan had sent an urgent message: everyone from the Academy was to come to Ruusan to join the battle against the Jedi. The apprentices were all to be presented with lightsabers and given seats in the Brotherhood of Darkness, elevating them to the ranks of the Dark Lords of the Sith.
"Why not?" she asked. "Qordis has given all the students the rank of Dark Lord of the Sith."
Seeing him wince, she stepped forward and wrapped her arms around his neck, looking up into his face. "Bane," she breathed, "we're going to fight the Jedi! We're going to join Lord Kaan's Brotherhood of Darkness!"
The constant battling of the Sith since the beginning of recorded history served a necessary purpose: it kept the power of the dark side concentrated in a few powerful individuals. The Brotherhood had changed all that. There were now a hundred or more Dark Lords following Kaan, but most were weak and inferior. The Sith numbers were greater than they had ever been, yet they were still losing the war against the Jedi.

- Darth Bane: Path of Destruction