are you talking about the Quilting thread? i would think a king sized quilt can easily use up 2000 yards of thread- i always use cones that hold 5000+ yards---and know i use up 1/2 a cone (sometimes)
of course it is all relative and really depends on your quilting style- how dense you are quilting- if you are using the same thread in the bobbin you will need twice as much--i don't know how much thread is on regular spools- haven't bought one in years- but guessing maybe 500 yds? if so i would think i would need at least 3- maybe 4 to be sure i wasn't going to run out before the end-
it may be best to keep a log at first- quilt a quilt and keep track of how many spools/bobbins you use.
i know with my long arm (which has large size M bobbins) it is normal for me to use 5-6 bobbins for a queen sized quilt- and i can thread a bobbin for my regular sewing machine from one of my la bobbins and get 2 3/4 filled bobbins- so that would mean it would take about 11+ bobbins in my regular machine to do the same quilting. your (location) says you are a 'hand quilter for hire'===do you know how much thread you use to hand quilt a king? i would double that amount- buy that much & give it a try- keep notes- if you wind up with lots left over---adjust your figures- if you run out ---
everyone quilts differently- density, batting/fabrics selection, stitch length...all play a part in how much thread it will take.so we would all have different answers-none of which may hold true for you.