Beware, much of this is based on inferences. I might make a revised version if I get around to it.
I dont know how many Jedi followed Revan off to fight Mandalorians, but lets say it was about half. Since I also dont know the number of Jedi at the time of the Mandalorinan wars, lets say there are 5000. There were about 10000 at the time of ep1, but because of the Great Sith War, we can safely assume they only had 5000, maybe less than that even. That means 2500 followed Revan and lets say about half died in the war(bear with me). so Revan has 1250 after the war. He presumably recruits a lot I think that because one of the Masters in KOTOR (dont remember which one) said that Revans forces outnumbered them. There were 5000 Jedi 2500 Jedi left in the Order after Revans followers left so lets say 1000 of them went to follow Revan giving him 2350 and the Order 1500. Stay with me now, Revan has his people on Korriban recruit some guys(probably one or two hundred) and gives him 2500 Sith against the Orders 1500.
In KOTOR you kill probably around 300 of Malaks Sith(Revans former followers). that still leave 2200 for the Order to fight. I say that the Order did rather nicely against the Sith.