Many medieval kingdoms had contested borders, I suppose if you were to lay them out on a map, you would have 2 nations (one east and one west) and the land in the middle could be claimed by both.
England claiming Calais in France (post Tudor I believe) would be a good example of this.
Try making up some history for the kingdoms. For example, Kingdom A captured a small patch of land from Kingdom B across a river. Maybe a queen from Kingdom C became a widow and married the king of Kingdom D combining the 2 kingdoms making it much larger than others. etc
There can be as many kingdoms as you wish on your map. There could be pockets of bandits, or primitive peoples occupying small areas. Its a blank canvas limited only by your imagination
Hope this helps