I think 18 is fair and should remain as is.
This just brings an advantage to clans that have a bigger active member count and considering the low population of the server, this will bring imbalance.
18 chars run by a single person is at a disadvantage vs 2 ppl with 9 chars each.
If you think there is an imbalance, pvp wise , on the server; there isn't. Some clans are more advanced in lvl/equip while other lack behind even though they have a higher member count. this will balance itself out in time and does not need intervention.