You can't Gain XP in servers under following circumstances:
1) You can't gain XP in private TDM server no matter how many players are present in it.
2) If the server is Public, it should allow at least 8 players (minimum) otherwise, you can't gain XP.
Regarding your Question, we don't require 8 people in a server to gain XP. The Limit of server should be 8 or greater than 8 to enable XP gaining in that server.
I was just playing in a server of limit 14 people maximum but there were 6 people in it. I could gain XP in it.
This is done to prevent botting/XP farming.
