How many hair pins?

I want to get these for my bridesmaids. They are wearing their hair half up, but I am not exactly sure how. I think they would put a few pins in their hair where it is pulled back. (like where the elastic would go.)

So, how many do I get per girl? They come in sets of 3.
3 seems too few & 6 too many.

bumping for possible help from the evening crowd.
Any suggestions?

Thanks.