Whenever I order or buy yarn for a spcific project, I always buy one more ball than the pattern calls for. I’ve had too many close calls.
Ribby in the 40/41 size calls for 12 balls of Peruvian wool. If you are doing it in 2 colours, the pattern says to get 1/3 in the sleeve colour and 2/3’s in the body colour.
I overordered and got 9 balls of the body colour and 5 of the sleeve colour. And I am very glad I did.

That little bit of yarn is all that remains of the first 4 balls. I did make my sleeves an inch longer than the pattern said, but I still think 4 balls wouldn’t have been enough considering that I haven’t done the neckband yet, and I have 16 more rows on each sleeve.
However I have 2 full and 2 part balls of the main colour. I think that for the 40/41 size, if I made it again I would get 7 balls of the main colour and 5 of the second colour.
But then I wouldn’t have anything to add to the stash afterwards 😉