Ideas and suggestions.

it would be better to introduce some other limitations

In the fifth installment of Heroes, there's a fixed number of creatures per mana unit. However, this isn't the best option. Introducing a coefficient that depends on the level would be a good solution.
For example:
10 creatures of level 1 per 1 mana
7 creatures of level 2 per 1 mana
4 creatures of level 3 per 1 mana
2.5 creatures of level 4 per 1 mana
1.5 creatures of level 5 per 1 mana
1 creature of level 6 per 1 mana
0.5 creatures of level 7 per 1 mana
Alternatively, it could depend on the growth rate, but that might be less aesthetically pleasing.