Angel - It depends. Choke selection depends on the layout of the shoot. Typically I/C and Mod will work fine. However, remember that since the bird will be flying towards you, your first shot will be a greater distance - therefore, you will want to use the tighter choke as your first shot, and in the unlikely event that you need a second shot
you will have the more open choke for the closer bird.
Pen-raised pheasants being shot from the front don't need big medicine. Modest loads of #6 shot will be fine.
Regarding number of shells - again, it depends. To try to get a guess on the number of shells, divide the number of birds being released by the number of shooters. That will be an approximation of birds you should be shooting at. Assuming two shots per bird, that would be a minimum number of shells. I'd double that number,
Remember that in many instances, after the driven portion of the shoot you will go out on a walkup hunt for those birds who survived the gauntlet. A chance to shoot another gun, and burn some more powder!
Enjoy!