We don't currently offer the option to add a hard maximum of members per plan. However, if you want to make sure that you don't go over a total member count, you can set your new members to require approval before accessing the space:
You can see a full total under Manage » Members » the number in parentheses on the "Send Email to List" button. You can also filter this list (by plan or company) to see the number of people on a particular plan.
If you want a more detailed view, go to Analyze » Members.
There you can see an overall total and a number per plan.