The Ouran High School Host Club manga is indeed complete. It wrapped up with all the storylines resolved and characters' arcs concluded. Fans got a satisfying finish.
In the end of the Ouran High School manga, there's a sense of closure and growth for the characters. They face various challenges and come out stronger, with many achieving their dreams and finding love.
It's hard to give an exact number. Different editions and reprints might vary. Your best bet is to look up on dedicated manga databases or forums where fans have likely discussed this in detail.
In an 'Ouran High School Host Club' OC fanfic, the OC can bring new dynamics to the story. For example, if the OC is a new student who has a different background or personality compared to the regular characters. This can lead to various plot developments. Maybe the OC is from a different country and has different cultural norms, which can create humorous or heart - warming situations as the Host Club members try to understand and adapt to the OC.