How to evaluate Levels on Pro Sites plugin

Hi, I'm working with the Pro Sites plugin and I need to code a little script to limit the number of users on the different Levels I've set up on Pro Sites.

For example for people that create a site using the Free trial I'll need to limit the access to just on user, for Level 1 let's say the limit is 10 users and so on... What's the variable I need to evaluate to know the level a site is using?

Thanks a lot!