The limitation is there to prevent exploitation by guilds; "Oh we have 3 rogues at the raid boss and they dropped a campfire let's port there." Similarly, if they increased the limit from 12 to 18 then that would be, "Oh, we have 3 rogues there and they dropped a campfire. Now the other 15 of you click over there and drop a guild banner; so the rest of us can banner over." Even if the, limitation was just at 16, the only change would be that they would drop 2 of the rogues from the fellowship after the campfire is up and add 2 others to get the 18 there. 12 accounts would be ok, but there is still the issue of what happens when I want to have my main in one fellowship with other people in the 105-110 range and my level 50 alt in a fellowship with a different set of people who are in the 50-65 level range? The real solution would be to have a similar clicky added to the game that works when 3 people in your group are in the same location; allowing you to port to them. The limitation on this would be that it would not work within raid instances.