I'm really not sure what you're saying. In my opinion, New Super Mario Bros. etc. are very linear -- you complete the stages one after the other. You can sometimes warp to another world or take a different route, but you can't skip over a stage if you don't like it.
Ukulele will have a hub world like Banjo-Kazooie. I.e. a Grunty's Lair, or Isle O' Hags, equivalent. You'll be able to unlock whichever world you want, providing you have enough Pagies.
Today it was announced that players will be able to expand worlds by using more Pagies, or move straight on to new worlds and come back to expand the previous worlds later by using the Pagies they unlock in the later levels. Alternatively they could 'spend' their Pagies in a more linear fashion and do the levels one by one in order. Look at this thread for more: viewtopic.php?f=4&t=341
In terms of the number of levels; I'd like about eight to twelve.