This seems like an easy answer to find on Google but I couldn't find it when I searched, so maybe somebody here knows.
One per flower, once the flower is fertilised the white part falls off. The back of the flower, the green part, then starts to swell if fertilised and that turns into your blueberry. I hand pollinate mine with an electric toothbrush for extra crop. If they dry up and drop off once the flower has finished they aren't pollinated. This is my bush that I have pollinated.
It looks like mine are slightly ahead of yours, I'm in the uk in a part that is equal to the usa 6a / 6b plant region.