18-21 strands of (original) Fast Flight used to the the standard.
Nowadays, the term "Fast Flight" is used both for a specific material and a generic name for most any HMPE material.
18 strands of the Fast Flight brand name is a one-size-fits-all string. It's just not practical for a store to try and stock strings built for specific draw weights--that's why there are vendors like me. I don't stock strings, I make them to order--too many different materials, colors, lengths, bow types, draw weights, etc. etc. etc. to try to stock a string for each one.
I keep several different types and sizes (and colors) of serving material on hand--another variable that makes it impractical for stores to try and stock strings to fit a specific bow. There's no standard for nock fit, then add different material types and strand counts...it can get pretty deep.
Then you have personal preferance. Some like the "skinny" strings, some fat, some in-between. 3 Rivers would need a small warehouse just for strings......
There's not a big difference between Dynaflight 10 and Dynaflight 97. D-10 is SK78 Dyneema, D-97 is SK75 Dyneema. 8190 is SK90, but still not a huge difference. D-10 isn't on the market anymore, except as Force 10 crossbow string material.
There's an FAQ section about strings on my site, with more added as it's brought to my attention.
Chad