Again, all else being equal, cannot see why number of VLANs should matter much, if anything at all.
When @MHM Cisco World notes more VLANs can support more hosts (and he also appears to assume an added VLAN will also be supporting a /24 network), well, often that might be true, but it doesn't have to be. For example, if I have a single VLAN hosting a /24 network, and I replace that with four VLANs each having /26 networks, the maximum number of possible hosts has actually (slightly) decreased. I.e. it's possible/likely data volume hasn't changed at all.
Further if you replace one network with four networks, you might increase the amount of traffic that needs to be routed, but it's also possible there's no change there too.
Often routers will be quoted as supporting X number of hosts, but that also generally assumes there's some expected traffic volume from those hosts, which could be far, far off from what your network hosts will, or will not, consume.
(One of my many stories I like to tell, is at one time, I supported an engineering departmental LAN segment, where our overall traffic was higher than found on our [a Fortune 100] corporation's backbone. Among our hosts, we has about 20 CADD workstations, which constantly updated CADD files on a server with every keystroke. Because of this, our LAN segment ran at the same bandwidth, as the corporate backbone, i.e. the only two LAN segments running at the higher bandwidth.)
What router specs (often/should) provide is the router's bandwidth and PPS capabilities. With those, and if we "know" what our network's actually aggregate bandwidth and PPS needs will be, we can calculate whether any particular router can, or cannot, meet our needs.
Without actually knowledge of our host's traffic requirements, we cannot really determine whether our router is of sufficient capacity.
So, again, determining a router's capacity based on number of hosts and/or number of VLANs (using number of VLANs, for such a determination, especially without even knowing networks' /#, seems, to me, an extremely poor estimation basis), is likely, very hit-or-miss.
BTW, a router's capacity might also be judged by how many (directly connected) ports, of specific bandwidths, it can support at wire-rate/line-speed. I.e. number of hosts doesn't matter, as ports will be ultimate limit of traffic that can flow through router. Also, BTW, as many small routers routing is dependent on a general CPU, they often cannot support all their possible ports at wire-rate/line-speed (in fact, in the past, some Cisco ISRs could not even support ONE of their ports at its maximum bandwidth).
