There is no set number of seats set aside for "saver" (or the low) FF level. The airlines only say it is a "percentage" of the seats on the plane. But that "percentage" can be as low as 0 seats (0%)! But normally, it may be more like 5 or so. But once those are taken, they are gone!

Remember - if the "saver" seats (25000 miles) are not available, there are always the "high level" seats (for 50000 miles)! If there are any seats available on the plane, you can get them for 50000 miles! (The airlines love this. Why give away seats for 25000 miles, when the FF can burn 50000 miles for the same seat! )