1- Every night you pray 20 rak’as (ten 2 by 2 salats), and it’s preferred that you pray 8 of them after Maghreb and 12 of them any time after Ishaa’. That’s 30 x 20 = 600 so far
2- The on the nights of Qadr you pray 100 extra rak’as each night. That’s 3 x 100 = 300, which means we have 900 so far.
3- Then on the last 10 nights of Ramadhan you pray an additional 10 rak’as each night. So the total would be 1000.