You can be a member of up to 5 player guilds (and all the NPC guilds) and joining is by account, not character. I am in a large trading guild and a small social guild. I may join a PvP and trials focussed guild later when I have finished soloing through the story, but for now I am good.
Thanks for the great answer. Just a quick follow up, what is an npc guild?
Right now you can join 3 npc (non player character, so computer controlled) guilds, the fighter's guild, mages guild, and the undaunted. Each one has a main quest line and they plan to add more npc guilds in the future.