How many ounces are in a yard of yarn?

This depends on the yarn.

To give you a comparison, here are 3 different weight yarns (from Lion Brand web site). As you can see, even weight is not a firm determining factor in figuring the number of yards, although I would venture to say in most cases a lighter weight yarn will have more yards to the ounce.

Vanna's Glamour® #2 sport weight

1.75 oz./50 g (202 yd/185 m)

Jamie® #3 DK Light worsted

Solids: 1.75 oz (50 g), 137 yd (125 m)

Amazing® #4 worsted

1.75 oz./50 g (147 yd/135 m)