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As long as each character has a reason for being there, or is just random npc X, like a class builder instead of a character, then no, I do not believe you can have too many characters. One of my favorites series of RPGs, Suikoden, is built around the concept of 108 stars of Destiny, which means there are 108 characters, now only about 82 of those at any time are available to be on your party, but each character has a back story and a purpose to join your army. This is a great way to do it. So as long as all 18 of those characters has a purpose and at least a small story, then it is worth having them all.
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