A yard of fabric is the width of the fabric x 36". You can look at the details of the fabric to find the fabric width.
For Example:
Sunbrella Canvas Air Blue fabric is 54" wide.? A yard of the fabric would be 54" x 36".
The fabric comes on rolls between 45 - 60 yards. When ordering you can order as many yards as you need. If you order a large quanity of fabric (40 or more yards) you may or may not be able to get it in a single piece.