TL;DR - unless you want to play different races/gender, or want to hide from your friends, there's no reason to play multiple characters.

You only really need one character, because this game is designed so that your character class/job is defined by the weapon you use. You'll need to enroll with the relevant class guild (i.e. do its quests) to be able to hold the weapon, but this game is based on the assumption that you have multiple classes.

So just go enroll at the Conjurer guild and you will be a conjurer when you equip a staff. Many people are already level 50 in all classes/jobs.

Also note, that at Lv.30 Arcanist, not only do you get to become a Summoner, you will also get to become a Scholar, which is also a healer. The Scholar job quest is by the counter in the same Arcanist guild room. The Arcanist is the only class in this game that will open up 2 jobs at the same time.