High density wi-fi is served best by many small cells with narrow bands. This avoids co-channel interference and over-crowding on the APs.
I would not try and solve a high-density requirement by using a few 8x8 APs here and there. You would be best suited just deploying many more 4x4 APs and reducing their transmit power and using 20Mhz channels to reduce overlap.
You would really do best to consult with a wi-fi certified Fortinet partner to do a proper site survey and predictive layout for your office. There are so many variables when it comes to wi-fi you really can't get all of your answers on a forum like this.