N+1 is the optimum number of bikes, where N is the number already owned 

Ah, cyclists in pairs always feel a bit sad when hearing stories of their buddies having to ‘convince’ partners that a new bike is required.

It probably is quite baffling for non-cyclists to understand how one person could need more bikes than fingers on one hand. Cyclists, on the other hand, see a partner’s purchase as a good reason to do some bike shopping of their own.