That's a very good question.and I see that you have already started expanding your arena.
But I would suggest to increase one type of seats at a time so that you can see which seats are filling out and which are not.
If you ask me I would suggest you that for the first 10000 seats,your seat distribution should be like
Bleachers-8000
Lower tier-1600
Courtsides-250
Luxury boxes-20
You can increase the lower tier to 1800 or even 2000 if you want..and to carry out the expansion,you should first build 5 luxury boxes and then about 250lower tier(to a total of 1000 lower tiers).followed by expanding your bleachers to close to 6000.then build about 50 court sides and about 7 luxury boxes.