For the set of natural numbers and the set of odd numbers, 2n-1 is the relationship that yields a one-to-one correspondence between them. For other sets, a different formula (not equation – an equation has an equals sign in it, as the name indicates) would serve the same purpose.
And I have agreed numerous times thar {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} cannot be placed into one-to-one correspondence with {1, 3, 5, 7, 9}. No one has claimed that it can, since one set has twice as many members as the other. On the other hand, {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} does correspond one-to-one with {1, 3, 5, 7, 9, 11, 13, 15, 17, 19}. You are hung up on stopping at 10 both times, when the question we are interested in has nothing to do with that but is instead all about how many members there are.
Usagi