Although up to 18 people may participate in a Garrison event, it is not recommended. There are a number of people that have had sucess with 18 people, but the more people there are, the more mobs that you have to fight. 12 people, forming 2, well rounded parties is an excellent setup. Each party should be similar to what you would look for in an xp party. Since the number of enemies is based on the number of parties, an alliance of 13 will have a much harder time than an alliance of 12.