Knitting -- skein count

How much yarn do you need?
Depends on lots of things, I have just one concrete example.
For Wool of the Andes or any other worsted, if you buy 16 skeins, 50 grams (2 ounces) per skein, you should be able to make:
1 boatneck pullover.  200 stitches around (40 inch chest), 25 inch length; 22 inch sleeves plus 8 rounds of ribbed cuff.  Size 7 circular needles on the body and sleeves, size 5 on the ribbing.
1 pair matching gloves for size 7 1/2 hands.  Size 3 double point needles.
1 pair matching crew socks for size 7 1/2 feet.  Size 5 double point needles.
A simple ribbed cap on size 7 circular needles for a head that wears a 7 1/2 hat.

About all those 7 1/2s.
They are the size of my high school class ring and the inches around my wrist, the size of shoe I wear (this is U.S. sizing, I don't know from European), and the size of English style horseback riding hard hat that I wore back in the day.
YMMV.

So how much  do you  need?
Children's to pre-teens socks, gloves & hat; one skein per sock, glove, and hat.
Teen's to adult's socks, 1 to 2 skeins each sock or glove.  The hat will depend on how fancy your pattern is.
Children's pullover probably less than 10 skeins.
Teen's to adult's pullover, 11 up to 20 depending on chest size and length. 

I think I said you should buy all the skeins together to as to get them all in one dyelot.
You might want to specify in comments on your order, that if they don't have one dyelot, they should delay shipment until they do.
When you get to the middle of a pullover and run out of yarn, buying more will probably get you a different dyelot.  I did this once and it showed. 
Experienced knitters who see me wearing that tee won't say anything because that would be rude, but they'll know and they'll nod sagely: didn't buy enough yarn before starting.
You can probably get away with buying the 2-4 skeins for the socks separately because they'll be hidden under your jeans and inside your shoes.
So why make the socks in a matching color?
Because when you get home you'll want to ditch your shoes but keep the socks on so your feet are warm.

I apologize for leaving this subject so late but with the pageviews building up on the knitting posts,
I am guessing you are all MUCH SMARTER at arithmetic than I am and figured out the adjustments
with the help of the gauge information for the yarn you decided to use.
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