Hi Shawtay!
The largest respawn of that I know is at Thais Bat Dungeon, here. There is around 21 bats.
And also at Edron in the cave of Sir Valorcrest there respawn around 12 bats.
Sources: Tibiopedia
I think that for now the best place to finish bestiary on Bats is the way to Drume on Bounac.
You will find there about 20 bats (probably 18, but I'm not sure), so I think this option let's you complete bestiary really fast and it's worth to mention.
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