if your quilt is over 90" your choices are limited. you can have multiple seams or you can buy wide backing.
if your fabric is 42" wide (and most are not 45" anymore) you need more than two runs side by side to get the width you need for a king or even a generous queen (long drop).
i need 9 yards for a generous queen (98" square*). that includes the extra on the sides for the quilting frame clamps. i lose a lot in the width in the beginning (about 16-18") and a little gets trimmed away at the end and becomes part of my stash. even on sale at $4.00 py it still costs $36 for the backing alone. i'm always on the lookout for that price or less for something i think i can use in the future as a backing. wide backing has to be less than $12 per yard with shipping to compete with that price. it's usually the shipping that tips the scales.
*i have a sideboard on my bed and quilts look odd if they don't at least hit the top of the sideboard.