To answer the OP's initial question -
You NEED 10 healthy OL every day in order to get through practice (5 for the offense and 5 for the scout team).
You also NEED 7 healthy OL to be active on game day (I suppose you could get by with 6 if your OL were very flexible and you liked living dangerously.)
Other than that, you don't NEED anything. It doesn't really matter whether OL 8-10 are on your 53 man roster or PS. However, OL is the type of position you can develop. So I would want (as opposed to need) all the guys we drafted who don't make the 53 (Larsen, Welch, Bussey, Ohrenberger, etc) to be on the PS.
So I can easily see this type of arrangement:
53 - 5 current starters, Levoir, Wendell, Kaczur, Ojakajima (whatever his name is).
PS - Bussey, Welch, Ohrenberger, Larsen.
That's 13 OL and I don't think it would be too much.