How do you determine how many flats of annuals you will need per square yard of garden space?

Most good Garden Centers will have a chart to tell you how many plants are needed to fill a given area. It will depend upon the recommended spacing for the given plant(s), that is different plants will require different spacing. Once you know that, just divide that number by the number of plants in the flat. A quick rule of thumb for spacing is to add the expected spread of two plants together and divide by two. Good Luck.

It depends on what kind of annuals you are planting since they vary in growth pattern and size- for instance, snap dragons are tall and narrow, one per 8 inches to a foot, but wave petunias can cover 3 feet or more. Can you specify what annuals you are considering?