Optimized Role-Playing Gaming: Number of Players

Tabletop RPG: 3-4 players is ideal, 5 is upper limit and 6 maximum overflow.
Okay to have just 1-2 players, but until have at least 3, the full social dynamics aren't fully possible for small group dynamics like "factions" and optimized complimentary team skillsets, etc.
Read about the application of Forming, Storming, Norming, Performing, Adjourning, and Re-forming relevant to Tuckman's Theories of small group formation, communication, and dynamics in application to role-playing games.