I supose it depends on how much you want to spend and how much you can appreciate the diferences.
I run quite a lot and own a few running shoes, but mostly I have two pairs (the rest are genreally "old shoes" that I keep for some reason or other). Personally I "need" one pair of shoes for running in the street and another one for trail running. I am used to running in the hills and mountains and I need a shoe that does not have me feeling every rock or slipping in the mud. Sadly, this shoes are a bit more rigid, so if I use them for street running they feel pretty wrong (I feel like I am having a fight with the floor ).
While street running I do speedwork as well as long runs. For me light running shoes work just the same wether I am running 5k, 10k or a half marathon.