How many freq flyer miles are needed to fly from Phil pa to jamica with American Airlines?

It depends. The lowest possible number is 12,500. That is for economy class travel, in the low season (Sept. 7 - Nov. 14 in 2013) and on flights where the initial allocation of award seats hasn't been used up yet (so book early).

Outside low season, it's 17,500 miles. To sit in the big seats in the front of the plane, it's 30,000 miles any time of year.

If you book late and all the initial award seats have been taken, you can still get an award seat for twice that number of miles.

Those are one-way figures. Double them if you want to get back to Philly. Flights in each direction don't have to use the same number of miles. For instance, you can decide to fly in coach one way but return in business class. Or, you may get a flight one way that still has low-mileage seats available, but you may not find one like that in the other direction.