In simple and approximate number
level 1 really only does less than 20.
aircraft carrying 50+ pax need a level 3 base.
Level 4, I haven't done one yet but assume the 150 seat Jets may need a 4 base, and if you do try to send (change base or purchase) an aircraft to a too small base Sami has built in a rejection which mentions the base as being 'too small'.
It does of course vary with different aircraft of the same capacity I expect, runway length and service facilities also being a factor.
Do note that level 1 and 2 have no customs facilities, so the base country only can be served.