Moving discussion from the comments here:

Hi, I set the number to 2, but AI still creates more than 2 species. I don't know why this mod not working...

Hi,
I can think of a few reasons and it could be any one of them:
1. Cause
Another mod is overriding my game rule or all gamerules in general

Solutions
- Locate the mod which does it and let me know so I can create a patch. There are thousands of mods on this workshop so you'll have to identify this for me.
- Move this mod to load after the mod which overrides my changes
- Move this mod to be the last in the load order without looking up which mod caused the conflict

2. Cause
Species were created using an event or via code by another mod

Solution
This is beyond the scope of this mod. This mod prevents species form being created by scenarios that trigger the game rule. For cases such as this I've created merge ai species mod.

3. Cause
Species were created by xeno compatibility

Solution
There is none. See last question in the FAQ for why. This mod is intended to cover this but it cannot at the moment due to game API not working. Not through a fault of my own.

4. Cause
An ai empire has migration treaty or recently has conquered anther ai empire

Solution
This increases the number of ai species in an empire but is beyond the scope of this mod. Use merge ai species for this.