Google Apps for Business customers can purchase more user accounts at any time. Here are the instructions.

If you have an annual pre-paid subscription:

1. Sign in to the Google Admin console. 
2. Click Domain settings > Account information > Buy more user accounts.
3. Purchase the license to create more user accounts, as needed.

If you are on the Annual Plan (post-pay billing):

1. Sign in to the Google Admin console. 
2. In the classic Admin console, click Domain settings > Subscriptions & Billing.
   In the new Admin console, click Billing.
3. Click the + (plus sign) next to Google Apps for Business.
4. In the expanded section, click the add licenses link next to the number of licenses you currently have.
5. Enter the number of additional licenses you want to purchase and click Continue.
6. Check the box to agree to the terms and click I agree. Update my contract..

If you are on the Flexible Plan (post-pay billing):

You don't need to purchase additional licenses on the Flexible Plan. Additional licenses are added automatically when you add new users.

If you're not using Google Apps for Business, you can view your maximum number of user accounts in the Admin console.