I don't create many heroes. 2-3 heroes are enough. If you create 6-8, there won't be much use in the early levels. If the map is large, you can create 4-5 heroes, and even then, just to have them sit in castles and add bonuses to the units. Well, you can level them up a bit, and then it will be fine.

Added 2 minutes later
Moreover, it's better to create one hero with a huge army, like Ossir. But if you create many heroes with archers and wolf riders, then they will all be weak. I played on a large map. My opponent created 5-6 heroes, and I had 1, and I defeated them all and remained with a large army.