That depends, a party bus might be able to accommodate 20 passengers but honestly it depends on the size of the people, how well the passengers know each other and the amount of space each person requires for comfort.
See the formula below which you can adjust. I would go a little lower just to be safe.
Small party bus rentals seat up to 15. For extra comfort, we suggest 12-14 Adults.
Small Limousine rentals seat up to 6. For extra comfort we suggest 4-5 Adults.
Medium party bus rentals seat up to 33. For extra comfort, we suggest 26 -30 Adults.
Medium Limousine rentals seat up to 10. For extra comfort, we suggest 7-8 Adults.
Large party bus rentals seat up to 40. For extra comfort, we suggest 35-37 Adults.
Specialty Limousines and Hummers seat up to 16. For extra comfort, we suggest 12-14 Adults.