Renee, I suggest that you purchase at least 20 yards of muslin--a few more yards if you can? (Mistakes happen) Somewhere Brenda Papadakis suggests 20 yards.
I purchased a whole bolt of muslin at Hobby Lobby before I began mine. Usually the dye lots are very close but you know, if you are gonna spend the time & effort--then do it up right ---are my thoughts on my quilts.
Look carefully at the muslin. There are all different kinds out there from some that have such a loose weave you could toss a bucket through them? You want good muslin!
You DO want to wash the muslin!(I launder all my fabric because the dyes often bother me) Muslin does shrink.
I really love muslin myself because I am a hand quilter. It quilts beautifully! Muslin works like any other fabric? I know some folks do not like it but it is more accurate for the time period, IMHO!
Happy Birthday on Sat to you!!!
Riley