For the Pfizer vaccine, once a vial is mixed with the diluent, there is a 6 hour window to use the 6 doses in it. Shouldn't be creating a huge amount of waste because of that, but it isn't unreasonable to expect that more than 1 vial would be prepared at a given time.

From the Prepare instructions on:

https://www.cdc.gov/vaccines/covid-19/info-by-product/pfizer...