Bid Day Shirts

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I feel like this is a really stupid question, but it's something I need to figure out.
I'm in charge of formal recruitment this year and I really want to get bid day shirts for the NMs & for the sisters. I was wondering how you go about having the shirts ON bid day...but since we don't know how many girls we will get, how do you know not to order too few or too many shirts?
It's my first year of formal on the "other side" so maybe someone with more experience can let me know how this works.

Thanks!