What is a ...of dragons? Form of measurement...like a murder of crows, gaggle of geese? Is it flight of drag

If you wanted it to be really unoriginal, you could say "A lot of dragons."

Or any of these, for starters:

A number of dragons

A group of dragons

A crowd of dragons

A multitude of dragons

A squirming, wriggling, mass of dragons

Depends on what style your book is.

Here is a link to a cool site called "A Beastly Garden of Wordy Delights", which has the name for many different groups of animals among which are -

Alligators - "A congregation"

Bucks - "A clash"

Buzzards - "A wake"

Cobras - "A quiver"

Eagles - "A convocation"

Finches - "A charm"

Owls - "A parliament"

In keeping with the "flight" line of ideas, I also think a "wing", "loft" or "Exodus" would work. Good luck! ; )

http://www.anapsid.org/beastly.html

If you want to get all fantasy-like, you could say a legion of dragons, or a throng of dragons.

Depends on the author you ask...

Anne McCaffery organizes them into "wings," "flights" and "weyrs." ("Weyr," however, is pretty much copywrite her.)

I've also seen "flock" and throng used. My personal preference is "den."

Or you could just come up with a term of your own.

legion