Great thought about the vehicle! We've got a 7 passenger van so we're set there. We originally bought it so we had a vehicle big enough to cart our Great Dane around.
The decision on how many kids we can handle is so difficult because we don't have any right now. My brother in law has 5 kids, ages 6-10, and we can handle them, but we don't live with them all day. They're not our kids. So the situation is so different as to the point it's hard to compare them. We feel that 3 is probably our limit, but it truly depends on ages of kids, too. The younger they are, the more difficult it would be for us financially due to daycare, etc. so that will be a consideration as well.