As many as will fit on my phone so hundreds. :)
When I was still collecting manga, I had roughly 60 volumes (all of Fruits Basket and the two Read or Die manga titles; most of Kare Kano; and random volumes for Ouran Host Club and Azumanga Daioh). Stopped collecting a few years ago and sold my whole collection off since I started caring more for anime than manga.
Bumping an old topic while I was out cleaning the Just Manga forum: moving old reviews and deleting any old spam threads.
If I was at my family home, I think I have around 50-75 manga volumes. It's because I used to volunteer at a public library to complete high school community service, you need 40 hours of community service in addition to GPA and other things to graduate high school.
I was tasked to buy manga volumes for the teenagers and children during my time when the economy was good and the library had enough funds to allocate for graphic novels (comic books, too.) Plus, they gave me old manga books that were tearing apart to me, leftovers that weren't donated to orphanages or schools.