When you’re on a call, press the Flash (or Phone/Talk) key on your phone to put the first call on hold.
When you hear a dial tone, dial the second number.
When you’re connected to the second party, press the Flash (or Phone/Talk) key again to complete the three way connection.
As for the hard limit...it depends on phone models. See if you can get to 5. Otherwise VoIP might be better for a 5 person conference call.