It's not always so much how many things you can do with a muslin as how many times you need to use one per day to deal with just one or two things! So mainly you get through a gazillion just clearing baby sick - about 3 times per feed when my DS was little, so pretty much one muslin per feed as he was sick a lot so 8 a day just for feeds! And then you always need one to hand as the baby will always be sick when you are on the move which means at least one or two in every room you go in plus one thrown permanently over your and his shoulders.
But here are some other uses, baby related or otherwise:
Cheap changing mat cover - keeps baby off cold plastic mat and can just throw in wash when poo goes all over it rather than having to wipe up the mat. Cheaper than those terry ones that John Lewis sell specially.
Putting in cot/moses basket under baby's head so when baby is a bit sick or drools or snotty when sleeping you can just remove muslin rather than having to change sheets every nap time.
Drying your face/hands when you haven't had chance to put a load of towels on for 3 weeks because you have a newborn...
Bib for baby/toddler/yourself when eating messy spaghetti...
Blowing your nose if you've run out of tissues or are trapped on sofa with sleeping/feeding baby
Draping over, ahem, strategic areas if you are breastfeeding in public, large of norkage and were dim enough to put on only a tight strappy vest top when you went out that day (happened to me several times - I don't learn quickly)
Sunshade if it suddenly gets sunny when you are walking in the park with the baby and you've forgotten to bring parasol/sunshade (or just weren't organised enough to buy them, in my case!)
Triangular bandage / wound dressing if you are in some sort of emergency first aid situation (and weren't organised enough to buy a first aid kit for the house, again as in my case!)
Squeezing out grated cucumber when making homemade tzatziki, as I did last week
Basically if you're disorganised like me they are a lifesaver.