OWC appears to have it listed as being able to support 64GB !! OWC Memory Upgrades For 21.5" iMac with Retina 4K Display (2019)
I would give them a call to verify they have in fact tested it and how fully. When you get into these larger memory configs many apps just can’t access all of it. You’ll need to talk with the app developer to see what they have tested. Buying this much RAM without a firm purpose I think is wasteful!
While a single app may not leverage the full RAM space multiple apps as well as the work its self might benefit from the larger RAM space collectively. As an example many video editing apps leverage scratch space on your drive. If you setup a virtual RAM drive you could use it instead of a physical drive (so much faster!) But! Be careful here. If your system looses power so does your data held in RAM! So it has its down side!