Ideally, you need at least 6 players; 3-5 players per team. But you could really play with as many players as you have. Certainly the more the merrier, and you can always play with ghost runners if you have fewer players.
1) Throw into opposite end cups for single, double, triple, or home run.
Ball misses cups or does not hit rim of cup = STRIKE
Ball hits rim, bounces off and is not caught = BALL
3 strikes = OUT!; 4 balls = WALK!
2) Once on base, steal by playing flip cup. Grab your cup at any time to start stealing. Complete flip before defender to steal base. You can steal as many bases as you'd like.
3) Score a run by stealing home from third, or being forced home by another batter.
1) Catch ball that bounces off rim of cup = OUT!
2) If the batter gets on base, follow them to opposite base. If they try to steal, flip your cup before them to record an OUT! Follow your batter on the bases until they score or you get them out.
3) Record 3 OUTS and you switch to offense.
1) If you fake grabbing your cup on offense, and the defender touches their cup, that's a balk. You get to take the next base. And the defender looks like a doofus.