If you have not max any N-girls 1000x N-Gacha will on average give:

15 SR-Girls
215 R-Girls
770 N-Girls

If you have max all N-girls (and are not level 50) 1000x N-Gacha will on average give:

30 SR-Girls
230 R-Girls
740 N-Girls

This is because when you get a N-Girls for a N-Gacha that is maxed there is around 3-4% chance that a R-girl or a SR-girl will be upgrated instead.