Since there are exactly 13 known stripes on the American flag, shouldn't it be LES and not des? I thought "des" was supposed to be used when it's an "unknown" or "indeterminate" number.
With "there are stripes" you don't state any number in English.
Also accepted "...le drapeau des états-unis".
Is it possible to say "Il y a de rayures" instead of "des rayures" if you are talking about stripes as a general category rather than countable objects?
No, that would mean "There are any stripes …", which doesn't make much sense.
Stripes in general would be "Il y a les rayures …", but they wouldn't fit either and would thus be misinterpreted as "the stripes".
Which American flag? Oh, you mean the USA!