One for every day of the week you train is ideal - but it depends on how regular you want to do washing! Also, its nice to have a choice of gi thickness; 1 judo gi is useful (and I think looks better), but other than that I get really cheap karate gis; they are thin (therefore cool), and you don't care if they get ripped. However this will depend on your training style! You can never have too many gi.
Unfortunately I think men tend to smell more than women, so I pretty much have to wash my gi after every session (and the judo gis take forever to dry).