I've seen a single VM handle 5000 users just fine (With Exchange). For Zimbra I can't imagine what the point of separating them out is unless it has functionality similar to DAG.
Right, many thousands of users from a single VM would make sense. Just give it more cores and more RAM until it can handle what is needed. Splitting out to another VM would only be useful if you are also adding more physical resources between the two as well, like one is on one server and one is on another and each have dedicated CPUs. Otherwise, the network connection between them just presents an extra, and unnecessary, bottleneck.
In my specific case, I have a cluster of hosts I could potentially spread the multi-server deployment across.
Still is only beneficial if the bottlenecks you have are addressed from doing so. Are you unable to give enough CPU or RAM from a single VM to meet the needs of the system? That's the only case that more VMs would be beneficial. Spreading out amongst physical hosts just creates network bottlenecks and OS overhead, otherwise.