Less than 160 Sidequests. In short: 1 Fragment equals 1 sidequests, though there are a few mandatory fragments. I can't point out how many of them ARE mandatory, so don't let me lie. I'd say 30, if I had to pick a number. Some fragments are just plain hidden.
And, no, there's no quest hub city. Unless you count Historia Crux, which is teh select screen on where you'd like to travel. Every place in the timeline has a few quests. The ganme is happy to tell you how many fragments are hidden in each area.