LOL, that's like asking how many sheets of sandpaper you'll need. Start with a full box of a dozen at a minimum.
The one bit of advice I give everyone about consumables while prepping for paint - do not skimp on the quality. Buy from an automotive paint supply house that services body shops. Not Walmart, not Harbor Freight, not some guy in a white van. Buy the best you can buy and don't worry about the cost. It'll cost a lot more if you have to reshoot the car due to issues down the road.
Yes buy a dozen to start with. When you open them, stretch them out and make a loose wadding with it and put it in a zip lock bag to keep it from drying out. You can un-ball them and re-use them up to a degree, use common sense. Once they are used a few times I save them for small parts. The used ones are also great to wipe all the masking paper down before tacking the body down. Also use the old cloths to wipe down about 6 feet of your air line before painting. I even rub the tack cloth on my arms and front half of my paint suit. I tend to stretch my consumables as far as I can but always use a fresh cloth when it comes to tacking the main body.