the head can hold many individual flowers so do you
mean the entire head or an actual individual flower?
if you mean the head it would be from a few to a few
hundred or even a thousand or more (some of my larger
onions get very large flower heads with many many flowers).
i haven't looked it up to know for sure so i don't
know if this varies by cultivar or type of onion.
my guess is that each flower of the common onions i
grow has six seeds.
now i'm going to go look it up. :)
i'm not having much luck on exact numbers so i will
pull one out of the air. :) it will be a multiple of
three. so three, or six seeds per flower seems to be
about what i can remember.
these two links have some interesting information: