There was too many when they added the fourth, three or less should be it.
Adding a 'new' tower isn't going to boost competition it's going to make it easier for multiple guilds to divide the space and peacefully AFK each at a different fort to hit 1,000 hours. Half the guilds with towers now are allied with each other. The less towers there is, the more competition there will be since guilds will have no choice but to actively attack/defend.