Your prepro should have 2 XLR outs and 2 rca outs for subwoofers, i.e. 4 sub outs already.
You can get great results with fewer subs if you place them correctly, understand how to create "virtual subs", etc. Eight subs seems willy-nilly (poor understanding) but, the swarm salesman would probably have more sales.
For a single listening position, you could get great results even with a single sub or 2 subs, if you have freedom of placement..
For more than 1 listening position, you might creep up to 4.The question you may ask yourself is..."Is taking care of your seat good enough? Or is the non-audiophile mother-in-law’s seat that important?"
(Nevertheless, you could also try some minidsp 2 in, 8 out, etc kit. Minidsp and dayton products have utility, but tend to be lower quality junk, not the same as a Yamaha, Sony, Marantz prepro)