True. Evey level up for seat capacity is done through seat upgrade, since it takes +3 days to complete them. Seat upgrades are given as middle level for whole stadium.
Between every level are 3 upgrades of seats.
You do not have to build seats piece by piece. Take big chunk 16 day build block or smaller 5-4-4 blocks. I prefer smaller ones since I get profit from tickets during that 16 day period of 1 block build.
21.000-31.000 4 parts of 2.500 seats 1st 2nd 3rd 5-4-4 days, so after 9 day i have 5.000 seats more 1 build block gives +10.000 seats after 16 days, and that is difference I prefer.