- Neutral creatures in Heroes 3

An Horde is 50-99 units. The fact that 30 joined is just luck. But it's better to be cautious and check their attitude towards you from the start. For the chance to get 30 dragons, you can even bring some thieves along (if you don't have the See spell, of course).

Yes, it's just about how many thieves you need to have so that 50 dragons join. <- The point is: the strength of an army depends on the number of units in it, and thieves are weak in combat. You can safely attack them with a starting army (provided that the number of them on the map is set by the computer, not a person) at any difficulty level.

Best regards, Draco