This may generate complaints about plot coupons (collect all the coupons and you get to send off for the ending!), which may be more or less justified according to the world-building. BUT --
One thing you DO have to do is hit them all. We were promised twelve worlds, seven fragments, four tests. Gliding over some is just cheating. Even if -- no, especially if -- you allude to the adventures involved in the ones not depicted. Show, don't tell. Even if you have to say that one world is a gray tower (at least, so it appears from the inside) and the visit consists of entering at the door at the bottom of the winding stair and exiting it at the top. (Hmmm. . . you could get a good creepy scene out of that as the characters try to figure out what the perils is here. 0:)
I suppose you could get around it IF you build in reasons why. Like having the character close a door on one world and blink, unable to remember what happened in there. Or someone mucks about with the magic so that it doesn't actually allow for twelve worlds. But you have to justify breaking the promise.