Can a Manga/Graphic Novel ever have too many characters?

No. You can make as many characters as you like! I read manga that has more then a 100 characters!! But thats what makes the storyline flow to what people want it to be. Like an example: If the arthur wants two characters fall in love there has to be some obstacles in the way for the arthur to show that they have feeling for each other so there has to be eniemes, anger, jelousy, and happiness to make that happen. So you need characters to lead them into it!
But characters are a good thing because there might be a character that someone in this world like and may continue reading your manga to see what happens to the character. Four characters are to boring or annoying if you have them in a manga it will pretty be obivous that the two characters love each other. People like mystery. Just make sure it dosent get too confusing. So you also have to keep an eye on that too.

it might confuse it a bit but as long as it does not get to confusing i think as many as you want is fine.

yeah! i once read a manga and it had WAY 2 many characters i was at a loss of what was happening, espesialy if the characters look simaler "but i thought she had that" "who was he again" "wait is this (name) or (name) they look the same" these are simaler questions i ask myself if that happens
-sorry 4 bad spelling