Before some jackass comes barging in saying "hurp derp here's a console command/add-on that will bypass the limit because I don't see any good reason not to set it to 99999," I'd like to remind everyone that vehicles get high priority when sending data to clients, and even then they're not exactly handled well. At one vehicle to a person, keeping 20 clients updated on the movements of 20 vehicles already has significant network issues (e.g. Driving near someone in speedkart causes collisions even though you don't see them come close to contacting, or getting run over by a car's invisible 3 meter long cattle catcher). If you have more people and need more vehicles, it's gonna become an even bigger mess. These cases will be an issue despite any case you might contrive that might actually call for more than 20, so please don't teach people how to break their game. The best solution to the limit is to design your gameplay around it.
