I only run 2 2TB disks on my ProxMox server. But it is a home server and not very extensively used. So in the end it all depends on your use.
But as @planet_jeroen said, as soon you are going to run a lot of VM’s and containers, Have very larghe mailboxes that need to be synced all the time for a lot of users, the best thing you can do, (maybe even before adding RAM) is adding disks/spindels so you increase I/O.