How to limit printing in a Vista workstation?

Hello.

It is a workstation with 20 PCs running on Vista Family Edition Premium.
There are about 40 users who log on with the same limited account and password.
The printer is a network Xerox WorkCenter Pro 128 PS.
I’d like to limit the copies each of the 40 users print to about 20 a day.
Please tell me what are the steps (sorry if this subject has already been discussed but I couldn’t find it).