Some computers may be able to handle many more than 20, but that is not taking networking into consideration. 200 vehicles might be fine for the host of the server alone with a supercomputer, but you have to consider other players, too. Twenty players means your game has to send the high-priority vehicle status updates to twenty different people. Those people also have some simulation going on client-side to compensate for lag, and it's pretty unlikely that everyone could handle them all, especially when those lag compensation simulations are working overdrive because the server isn't sending out updates because it's spreading itself thin between the nineteen other players.
Twenty is a reasonable limit, often enough for pretty much any standard server. You should try to work around the limit, not through it.
