CALs - per user - per device

hi all

from a previous question i learned that i need to purchase some additional CALs.  I was receiving the message below.

No license was available for user PIGEON\(user) using product FilePrint .  Use Licensing from the Administrative Tools folder to ensure that you have sufficient licenses.

License usage for product Windows Server is at the maximum number of per server licenses purchased. New license requests may not be granted. Consult Licensing from the Administrative Tools folder for more information.

when i checked our licensing info it gave me this:

installed - 5
max usage - 13

we have 19 users (employees) and 13 computers.  sometimes they are not all on the network and most of the time they are.  thats when most of the problems occur (some users cant access email, shared netword folder, etc)  

correct me if i am wrong, but in this situation will it be best to purchase additional CALs (per user)?

if so:

i read that it is not a good idea to mix per user and per device licenses on a server.  i did not build the sever and dont have any information on the licenses installed.  how can i find out if the original licenses were installed per user or per device?

also how do i reassign or assign CALs to new employees after others have left.  there have been several people who have come and gone in the past year and i havent reassigned any cals.  just deleted or added new users.

please ask me to clarify if necessary,  not to familiar with some server issues.

by the way we are running sbs 2003.  thanks.