How many yards of concrete is need to do a 12ft x 12ft patio?

This needs more information to supply a "best" answer. A 12ft by 12ft patio gives a surface area of 16 sq.yds. A yard of concrete is a cubic yard. The number of cubic yards depends on the thickness of the slab, and the size and depth of foundations (if any). This depends on the type of soil and subsoil, and whether you have very cold winters. If you live in a warm area, 3 yards is probably about right. If you are planning to do this yourself, try asking your local building inspector for his advice.