X Amount of Players condition ?

Hi,

I'm trying to make a mob which gets stronger depending on how many players are in X radius. I know how to use the levelmodifiers, more or less, but I don't know how to get his skills changing.

One of his abilities summons 3 strays when normally fighting against 1 player, but I want him to spawn 4 if fighting against 2, and 5 if fighting against 3. It would be so much easier if there was a condition, but I didn't found it.

So, please help me !