Some more info is needed before we can answer. Do you mean Elite Clan challenges? If so, all stats for the challenges are based upon any accrued leaderboard stats during that time (challenge start time - finish time), so any kills/whatever you get, should
count towards that challenge.
Elite clans do not have a member limit, that I'm aware of
If you have a bad connection, sometimes it will not show or give you credit for kills you know you got. I've had several games happen where literally every kill I get is counted as an assist.