Depends on the location. Springfield and Little Rock can stand to have only 3 or 4 to cover the entire area without putting too much stress on any one person.
I was really exhausted before I found Jeremy through a friend of mine. I did all my promo in an area that I worked, but the amount of walking, talking, and pure promotional work wore me down.
In bigger areas like Denver, I can see the street team being bigger, but from my experience, a little free time and a couple friends you can trust goes a long way.
My friends work hard for me on behalf of the label, so I've got it pretty easy.