I did some looking around at the beginning of the season for teams who promoted to D.III with complete arenas as I still think I will be promoting to D.III as well and I came up with this for mine:
Bleachers: 12-14
Lower Tier: 35-38
Courtside: 90-100
Luxury: 700-800
I ultimately decided on this:
Bleachers: 13
Lower Tier: 35
Courtside: 90
Luxury: 800
I feel like these would be pretty good for your as well but you may have to tinker with the Luxury boxes some as I have 50 and you have 20. Besides that, it should work pretty well with your promotion boost and arena size.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs