A few things I get asked a lot are: 1) How much fiber do I need to make a skein of yarn? and 2) How many skeins of yarn do I need to make a scarf or other project?
To answer #1)- Well, there are a few ways to measure this. First, it depends on your spinning equipment, method, and yarn size/weight. A jumbo bobbin on most spinning wheels can hold 3-4 oz of yarn. Let's assume you are spinning on average a 3.5 oz skein of bulky yarn on a wheel (not a drop spindle). So you should buy at least 3.8 -4 oz. of fiber per skein to cover any loss from short fibers or VM or anything else that falls out. You might want to keep a little extra ends for future reference or swatches so that should cover it.
To answer #2) - One way to guestimate how many skeins you'll need to knit a scarf, for example, is to weigh a similar item. I've weighed my scarves (yup, I did!) and they range from about 4 oz. for a sport weight skinny scarf to 2 lbs. for a super bulky Dr. Who type confection. Sweaters can weigh 6 lbs. You can probably make a baby hat or skinny scarf with one skein. Consider buying an entire medium fleece if you are making outerwear.
Bottom line is, make skeins in pairs and weigh everything first. You'll want the second skein, I guarantee it.