This news was sent by Google in April 2011. But, I think I have to keep this info also for myself. In short: There are only ten accounts created under a single Google Apps Free account (previously, we can have up to 50 accounts). In last April, Google Apps account holder should received an email from Google Apps Team informing this changes. Yes, I got some too — since I maintain some accounts using Google Apps:

We recently announced upcoming changes to the maximum number of users for Google Apps. We want to let you know that, as a current customer, the changes will not affect you.
As of May 10, any organization that signs up for a new account will be required to use the paid Google Apps for Business product in order to create more than 10 users. We honor our commitment to all existing customers and will allow you to add more than 10 users to your account for ___________.com at no additional charge, based on the limit in place when you joined us.

This new policy will work for accounts created after May 10, 2011. It will not affect existing accounts (created before this new policy). Google offers some flexible pricing scheme. Google for Business pricing and features:

• $5/user account/month OR $50 / user account / year
• 25 GB of email storage/account

I myself only have less than five users under my Google Apps account. Considering the features, I think the pricing scheme is still reasonable.