it depends on the type and model of the car it can vary from 2 to about 8 people except if you have a limo :)
4 Passengers can legally fit in a Toyota Supra (MKII-MKIV).
4 if you have a back seat
On a BMX bike my friends and I have fit 5. Two on the back pegs, one on the handlebars, one on the seat, one on the top tube. But if you really wanted to I'm sure you could hook up maybe a tandem...
four people can fit in a 240sx fastback cause there are only 4 seatbelts