You can attend as many sessions as you like!
The Australian Government Department of Health recommends that you should - “Do muscle strengthening activities on at least 2 days each week” and “Be active on most, preferably all, days every week”.
http://www.health.gov.au/internet/main/publishing.nsf/content/health-pubhlth-strateg-phys-act-guidelines
So each week you should aim to achieve a good mix of:
anaerobic and aerobic exercise
strength training for the upper/lower body and core
with a day’s rest in between high intensity sessions.
Make sure you purchase more sessions passes when each pass runs out.