There are currently 50 states that encompass the United States, these are bound together in a union with each other. Each state holds governmental jurisdiction over a defined geographic territory and shares its sovereignty with the United States Federal government. For example, in the trucking industry each state has its own regulations in regards trailers. Some states allow Doubles, some allow Turnpikes and some don't. To view all the state trailer restrictions, see Long Combination Vehicles.
When asked if there will be groups of states at some point, SCS Dev Eryk replied that, "...there will be packs of states when we reach states that are too small to be released as single dlc."