How can I check in Rules if the number of members is below 50 or not?
You need get the count of the total users so far from the users table.
To do that, you can use a condition with php code where you have to write a db_query("SELECT COUNT(*) FROM users"); //[not tested] 

if result gets number of existing users. you can return true/false to go to the action in order to assign points.

Let me know if you got it working.