If you are making your own extensions, you’ll have to go by trial and error. It will only take about 3-5 extensions before you have a good idea of how much hair you’ll need to make an extension that is X thick.
If you are getting extensions made, give the person making them a range. If you have one in particular that is a lot larger or smaller than the others, give them an estimate of the thickness of that one (or two). Then give them a range of sizes. “The rest vary between 1cm and 2cm thick, so if you made a random assortment between those two sizes, that would be great!”
If you are buying your own kanekalon, I extended my hair with about 1.5 packs and wound up with extensions that were 14-16 inches long. If you want longer ones, you’ll need more kanekalon. Kanekalon is cheap, so buying a bit extra is always a good idea. 2-3 packs is safe for a 14-16 inch full-head install, and perhaps 6+ packs would be good for a longer install than that. I hope that gives you an idea.
I hope this helps you!