How could i calculate the occupancy in a call center ?

I suggest that you search for "Earlang C" or "B" theories. Beside that formula, "occupancy" you will also need another formules; like for example:

Formula for the number of call receivers:

Number of call receivers = a

Central call demand= d

Clients row probability= c

Check the formula1 in the attached image (not possible to attach...I don´t know how to send it)

Earlang C concept is what is common refered to minutes traffic. For example, if you´ll receive hundreds calls/1hour, each one with2 minutes long, at the end we we´ll receive minutes or earlangs of traffic calls in that hour.

There´s another formules, you can also check the "offered load" and "traficc call intensity" formules.

All those will help you to find out the values that you´ll need.