The most honest answer for you is “it depends”: Given that your site only supports 24 users, so far, then , for THAT site shold be no more than 1-2 servers: One for Your database, one for your web/email/DNS/applications.
For your VOIP, you COULD go the full-blown Unified Communications path. Since you only have 24 hosts, Cisco offers the UC500 series, which can scale up to 52 users, provide routing, switching, even wireless in one box.
For the deployment across two floors, it would work best to have a patch-panel and/or second access switch for your users to be cabled into.
Without more info, though, it’ll be hard to give an exact list of necessary equipment…