The Los Angeles Police Department's Special Weapons And Tactics division use Ford Explorer SUVs:
This is the same for the New York City Police Department's Emergency Service Unit:
Here's a Chevrolet Tahoe used by the Boston PD's SWAT:
Not only SUVs are used. But also saloons. As seen here when an LAPD SWAT team in a slicktop Chevrolet Caprice confront a suspect armed with an AK-47 as seen at 3:28:
This vehicle only had three officers in it but you can tell they're SWAT due to their weaponary and clothing (helmets, vests...etc. and the fact that one guy is wearing shorts, which is fashionable amongst SWAT officers in the USA don't ask me why).
My point is that much like the UK, the USA also uses regular cars for SWAT teams, and these cars CANNOT carry six people.
This is why I suggest either there's a SWAT truck and a SWAT car, or you limit the personnel on the vehicle to a minimum of one person with a maximum of twelve people.
Twelve you say?
It all depends on the vehicle and the department. Some SWAT trucks will carry less crew depending on how the vehicle is laid out - if you have seats you will have a specific number.
But a lot of SWAT trucks have benches (pictured)
[Blocked Image: http://images.onset.freedom.com/ocregister/article/n0uwwt-b781217958z.120140211161434000gha1hv3tg.1.jpg]With benches you can fit a lot more people on them.
There's already two people in the cab of the vehicle, or four people depending on what the cab is. There will be a good six or eight more in the back. That's of course if you're responding with a full turnout.
Sometimes they only have one guy driving the truck and four in the back - that'd be five personnel.
This video shows SWAT trucks with more than six crew - they even have the officers hanging on the side and the back, with a dog too!