I've never found horses to be a reliably method of powering a server. They tend to fluctuate in their ability to maintain peak power.
Plus they crap everywhere.
Your question is hard to answer. As Hank said, its not about what ServUO can handle, there is no real limitation there. It comes down to the strength of your server and your amount of bandwidth. Then, you have to consider things like: how many items in the game, how many gumps open, etc.
As far as 20 connections at once, that doesn't require much at all and most decent home computers can handle it with a decent internet connection (not satellite or rural wireless). Any entry level VPS can handle 20 connections as well.