Between 5 and 30 members.
I've been in an Open class colorguard for the past 4 years, and the most members we've had in that time was 18. The least was 8. In 2005, Brigadiers had 6 members in IO, I believe. I remember Onyx having fewer than 10 in 2004. I think Interplay (IO) was under 15 that year. Kings HS in SO had 8 that year. Croatan had 6 in 2005 in SO, and 10(ish) in IO in 2006. Signature 2005 was comprised of around 13 members. I could go on for a while.
I definitely think colorguards average 15-20 members. There are certainly quite a few small teams representing the Open and World classes. Top world class teams certainly average in the upper 20s, but the rest of the colorguard population does not.