In Heroes of Might and Magic III, the number of units you can take from a Hive depends on the Hive's defenses. Kostya Derti has already written about this. There shouldn't be any problems inside the Hive itself; the key is to eliminate the defenders (if there are Angels or similar powerful units, split them into individual units – this helps a lot). Try it out, and with experience, you'll get the hang of it. Good luck.
