Hi Sandra,
Agastache and all plants are programed to make up to a certain amount of seeds per flower. That doesn't mean they always do. If the flower doesn't get pollinated, the weather isn't right, and probably a few other reasons, there will not be as many seeds. There can even be no seeds, or nonviable seed. Also, some varieties of flowers seem to have seeds fall out more readily when ripe, so it is a good idea to check them often to collect seed.
Hope this helps,
Remy