gawd I forgot what a beautiful computer that was… that gets lost in all the chatter over what a problematic model it wound up being…
Pretty sure it does not require pairs, in fact I believe the base model shipped with 3 4gb modules. The important thing is that you stick to the same speed, it can support 1333mhz or 1866mhz, you can't mix those.
There should be a bit of a performance boost for installing in pairs, though.
Technical specifications for the Mac Pro "Quad Core" 3.7 (Late 2013). Dates sold, processor type, memory info, hard drive details, price and more.
everymac.com
OWC is going to be a very good resource for compatible chips, although I bet you could find some dirt cheap on ebay.
Easily upgrade the memory of your 2013 to 2019 Apple Mac Pro up to 128GB. We also provide refurbished Macs, SSDs, external storage, docks, and more!
eshop.macsales.com