I'm using a Dell Precision 7910 Tower, which has dual cpu slots and 8 ram slots per cpu. I'd found a refurb on ebay with dual v4 xeon's cpu's already and minimal ram as a starting point. I'd added 8x 16gb samsung sticks with it, finding refurbs on ebay at the time for around $56/per stick. I can get 32gb sticks, but they're super expensive still used even ($300-400ea).
For windoze images, I tend to give it 8gb of ram, as I do some complex visios that have gotten cranky with less. Other linux vm's are usually pretty minimal, as they're pretty purpose-built, but spawning vendor appliances from f5, palo alto, or fortinet tend to all want 4-8gb of ram as a start. Throw in GNS3 routing instances for cisco things, and it goes quick there too.
Libreoffice gets ram-hungry with some formula spreadsheets too, having seen it using ~20gb at times as well with some of the more huge price lists or interface config generating forms I use.
I'd love to know how folks get by on 4 or 8gb of ram these days...
-mb