how much memory can a late 2013 Mac Pro hold?

Do not obsess over "all modules the same size". Feel free to upgrade by the each DIMM. A modest speed-up is provided in each matched pair, but not worth using any sleep over. Your 16GB modules are RDIMMs, and larger ones are RDIMMs also, so no mix-and-match issues for you.

Much more information in this follow-on article on the same site.

https://everymac.com/systems/apple/mac_pro/mac-pro-cylinder-faq/how-to-upgrade-mac-pro-cylinder-memory.html

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looks to be 128 GB.

https://everymac.com/actual-maximum-mac-ram/actual-maximum-mac-pro-ram-capacity.html