Both the center cell and the other 8 cells have their own, best symmetrical environment. It doesn't matter so much if the ratio deviates a little bit from 8 (e.g. 7.9): the whole band gap circuit has to be adjusted anyway.
If you need more BJTs anyway, you could surround the 3x3-cell block by another 16 "dummies" to create a 5x5-block, thereby guaranteeing a ratio closer to 8.0 for the inner block, for a nearly 3-fold greater area cost, however. And more difficult routing, of course.