We did one with that number of people - and just asked people to wear pastels (on top). They were all different colors, but that was OK, as they blended, and since they were all pastels - they did not clash. (We did not specify the bottom halves, as with the kids, they were not that visible.)
This also meant that no one had to go buy anything. :)
We also did one where I specified that everyone should wear a green top. We all matched, except my FIL who had a teal version of green - much darker than the rest... Oops!
We also did one where the boys/men wore light blue, and the girls/women wore light pink. That worked, although my SIL wore navy blue... it worked out, at least it was blue or pink. :)
My opinion is that solids work best, although if the colors are not too bright, you could have the kids wear some prints/stripes - you just want to keep the focus on the faces, not the clothing. :)
Good luck! Our biggest challenge has been 1 & 3 year olds. The 1 year olds want to walk out of the photo, and the 3 year olds make faces! Always fun though!