If you were going to do the middle size that takes 1200g then that's 12 x 100g balls. Each 100g ball is 200m long so you need 12 x 200m = 2400m
Your chosen yarn is 75m long so to work out how many balls you need divide 2400 by 75 = 32 balls. I'd see if I could lay aside a couple of extra balls just in case as there is nothing worse than running out of yarn.
The small size will take a ball or so less, the biggest size will take a ball or so more.
That's assuming you work to the correct tension and the yarns have the same wpi.
Thank you Woolly, thank you thank you thank you! That is clear and easy to follow and has cleared my brain fog (would you believe that I studied maths for some years with the OU and actually did quite well.....)
I am going to do all the given sizes for each of the 4 yarns, just to get the hang of it. While I'm doing it, I'll also discover that my favoured yarn will require over a billion tiny balls at a cost of £19.03 each, and the whole thing will become unaffordable and I'll do it in SSDK or something.......