It really depends on the towncar and driver as well as the size of the adults & children. Most towncars have the ability to transport 6 - the driver and two people in front, and three in the back. That is the number of seat belts. Most people do not want to share seat belts (and it isn't safe to do so). IMHO, 5 passengers will be very tight. Some towncars only have two seatbelts in the front or the driver uses the front middle seat to transport all his/her "stuff" needed for the day. They don't usually like to have 3 people in the front seat.
You also need to consider the amount of luggage as some parties of 5 will have more luggage than trunk space (especially if you are traveling with an infant car seat and a stroller).
The best thing to do is call the company you are considering and ask them. Several of the towncar sevices will send a van for the same price or just a few dollars more.
Enjoy your trip!
