Better to try aim for 120 Slots with Medium Size Siege Map as a minimum when making a Siege Event, the average applying Regiment likes to bring between 10 - 20 people so by having 120 Slots you'd accommodate for 5 - 8 Regiment depending on their overall size. This can allow you to take 1 Arty Regiment per Team, 1 Lights per Team and the rest as Line.
This can be done as a 70 vs 50 style Balance. 30 vs 20 would only really hold 3 - 4 Regiments, having Arty on each team is going to take 10 - 20 Slots from the offset, leaving only space for a small line or lights on 1 Team and probably 2 small line or lights for the other.