Original: Also according to the letter in W6 it says that there is a whistle in W3 (that is the water world).I didn't take time to find it because I am too lazy but I guess it has something to do with the boat... Also I think I remember watching a video with a warp whistle and it was different from the well known ones. This adds up to five warp whistles if the two iI was talking about were unique
Edit: So in the NES version it says W3 but actually it meant level 3 in world 1 (I played Super Mario Advance 4 to find that out) which was already covered and the video I was talking about was the secret box glitch which only works in the Japanese version. Here is the video
So really including glitches you can get 4 warp whistles.