Is there a certain amount of rounds Novak needs to finish ahead of Nadal in order to keep his number one ranking? (Besides the scenario that Slasher1985 shared??)
It's very easy.
If Nadal doesn't make the USO final, Novak will keep the #1.
If Nadal makes the final, Nole needs to make the QF to keep the #1.
If Nadal wins the title, Novak needs to make the final to keep the #1.