As doikor said, it’ll halve your RAM speed. That’s not a rule, but it’s typical.
For example 64gb from Samsung for 350€ https://www.dateks.lv/en/cenas/atmina-ram/1028642-samsung-64...
I think they go up to 512gb per single module these days. The really big ones are really hard to buy for us mere mortals but you can get them through various data center/enterprise hardware vendors.