Unless a specific number of extras are requested by the writers, 2 extras are normally booked for PPV’s and 4 each for the Raw and Smackdown tapings. Some weeks extras may only be booked one day in the future.
The extras that are booked may get matches or an opportunity to work out in the ring and be evaluated by Jamie Noble. Regardless, the information on talent evaluated is now loaded into a data base and the decision on whether the talent will be booked as extras again is made based on the evaluations. Good evaluation, and you are booked again…bad evaluation, not so much.