Whatever width you prefer ... audition several.
Plus remember that sashings do not have to have cornerstones ... it can be all sashing.
And too, there's no saying that it has to have sashings ... samplers can be done with the blocks side-by-side.
As for how many ... it all depends how large you want the finished quilt!
A queen size can vary immensely ... how much drop you want for sides and foot, and whether there is to be a tuck around pillows. Plus some prefer the blocks only on the top of the bed with extensive borders; some with blocks on both top and sides with minimal borders.
Measure out what sizeyou need and then do your calculations.