Generally, Google Apps allows all organizations to create unlimited number of users. If you're the an existent free user of Google Apps then you might be experiencing almost unlimited number of user creation. But now Google is going to limit, and will not allow to create more than 10 users for new customers.
Right from May 10, any organization that will signs up for only Paid Google Apps for Business, will be allowed to create more than 10 users.
But if you are an existing free user of Google Apps then, this change will not effect you and you'll be not charge for the 10+ users, based on the limit in place when you joined it.