The discounts are up to 40% off, and are often limited to certain resorts, and/or certain dates. The current discounts are 20-40% off for bookings 3/9-6/14 but it's very limited. The previous discounts were released in fall, for bookings through 3/8, and it seemed a much wider range of resorts were offered. I was able to get BCV & Poly in early March for 40% off, but only got 30% off for BC in late March.
You can book 1 room per AP holder age 18+. There have been many discussions on this on The DIS. I guess previously you could book up to 3 rooms per AP holder, but it has recently changed. Unfortunately people have been getting conflicting info from CMs.