The number of friends you can make is depended on the level you are in. Friend slot will increase for each level.
Every friend that you make will give you extra 3 parameters to distribute into ATT, DEF or Energy. So, make as many as you can!
Friends also useful during boss battles.
Summoning Friend: For each boss battle, you can summon friends of each element type that you are not. So, for example, I’m a water element fighter—so I will want to search my forest and fire friends and have them help me battle this foe. Note, you just need to add the players to your friends list. They do not need to be online to assist you in battle.