There were three you could get in quick succession, I'm pretty sure all of them in World 1, and so long as you cleared World 1 first, they could then take you straight to World 8.

EDIT: Okay, 2 in World 1, 1 in World 2. But if you claimed both World 1 whistles and cleared World 1 normally, then you could use both whistles to go straight to 8. Using the first in World 1 would only warp you to Worlds 2 through 4, but using it in World 2 would warp you to Worlds 5 through 7.

There were three you could get in quick succession, I'm pretty sure all of them in World 1, and so long as you cleared World 1 first, they could then take you straight to World 8.