Can one person play more than one Spirit?
Practically speaking, sure. Because the game doesn’t usually have secret information that only one player knows, one person can play as many Spirits as they’d like. However, that one person is taking on the roles of multiple Players, e.g., simultaneously being the Red Player, the Blue Player, and the Yellow Player. The game mechanics scale with the number of Players, not the number of people who are participating.
Two scenarios involve secrets and thus get easier if a person plays multi-handed: