Ok, speaking as someone who uses cable ties on a professional basis:
1) Don't buy the cheap ones. They just snap.
2) The Most useful sizes are 300mm (12") & 100mm (4").
3) Don't buy them from a D.I.Y. store. They see you coming. Buy them from a trade outlet.
4) Buy them in packs of 100. You will be surprised just how fast you go through them.
5) Your absolute minimum is 2 packs of the big ones and 3 of the small.
6) Cable ties are like 550 cord and bank line - the more the better and however much you have, you still don't have enough.