Board Thread:Lore/@comment-97.125.181.26-20150621121221/@comment-26298013-20150621210332

It never states exactly how many Balrogs there actually were, as far as I know. But several die to the hands of Elves, Eagles, and Men, and more almost certainly fall to the Valar when they destroy Thangorodrim. Durin's Bane survived, and it is possible that more Balrogs fled the destruction of Morgoth. Also, they would've used the tunnels that Gandalf mentions when describing his chase of Durin's Bane-the Dwarves didn't find the Balrog until after they built the Endless Stair.