Good luck on promotion! If you do promote, it will be to one of the 16 division III leagues in your country. Beyond that nobody can say for sure.
As for the number of seats you should have (and pricing) you should look at some of the teams in the division you're interested in who have been around in that division for several seasons. If you look at some of the more successful teams you should get an idea of how many seats they're selling at what price. It's hard to say X number of seats because every team has different success levels and so sales will vary somewhat, but looking at the arenas of managers who seem to know what they're doing should put you on the right track to make an informed decision for yourself.