I have seen (not grown myself) cosmos over 10 feet tall in a crowded dense "forest" of cosmos. Cosmos have a great variety of plant habits.

If I weren't already breeding zinnias "full time", I would be breeding cosmos. My first objective would be to cross the yellow ones with the pink ones. They have great potential for different flower forms, including double (Double Click) and petal variations (Sea Shells).

ZM

I have had similar, most 3-4 and some 6-8 on several occasions. I used seed I saved from previous years and do not recall where or when the first were acquired.

ETA: I vaguely recall my taller cosmos may have been mostly near the house. Probably just happenstance even if my memory is correct.

I think the Guiness Record cosmos is 12ft 3 inches

There are various species, and then cultivars of Cosmos. Generalizing may not be very productive.

Out of a pack of cosmos seeds there are always a few that have the genetics to grow super tall and take forever to bloom, but there will be 100 flowers on that plant. There are variations in seeds just like with any living thing. Enjoy the variety!.

By the way, a cosmos plant laid on its side along the soil will produce roots along the stem and branches that go upward from that, producing many more flowers. May be too late now, but try it sometime. Fun with flowers!

Susan's post reminded that my tall ones bloomed late. Frost was forecast and I took some as cut flowers. More blossoms opened and roots formed on the cuttings.