Windows small business server 2003 CAL question

We have a windows server for small business 2003 which shows there are 5 licenses installed in the Administrative Tools > Licensing, also it displays a maximum usage of 12.

The question is, how come 12 users could use the server with 5 CAL? Or what happens if more users that the number of CALs i have want to login?

There is already about 20 users created in active directory users.

Vad,