Go to Crucial's or OWC's site, put in your machine specs, and buy what they specify.
I suspect that you won't see much performance improvement (if any) with any more RAM than 16 GB. I don't have test links at hand, but they are pretty easy to turn up. Since you already have eight, and OEM memory is relatively cheap, put in 16 more. If you happen to have 1 x 8 or 2 x 4 lying around, use it and spend your money elsewhere.
After 16 GB RAM, I'd put money in, first, an SSD, and second an upgraded GPU. The SSD will bring much more performance boost than increasing RAM.
Fwiw, I think the "must be pairs" is from old PC's. I'm pretty sure (= don't rely on this) that current CPU's will address memory in any configuration (and I think Mac's have been able to for a long time). OWC does recommend installing memory in matched pairs "for best performance".