Unless you do MTBing or TTs etc you really only need one, lots of cyclists keep at least 2 a summer and whats called a winter bike used for cycling in not so good weather.
Its very easy to keep adding to what becomes a collection, buying a different frame material or one with different gearing or brakes etc but most bikes are designed to last a life time or a lot longer even if used 365 days a year,know any of my bikes should/could out last myself.
My own view is if you have space and the money and want another then why not, all my bikes are road bikes, sold or gave away all my vintage and MTBs and am trying to just keep the 12 left without adding any more, even if i do see another i would like, have to consider where will i keep it (no room) so if i buy it then i would have to part with one i already have.