User input only takes the last number

[Larz60+]

you want to collect seven pints of blood (not from the same donor, I hope)

so you need to accumulate the number,
so change your loop to accumulate the input value:

pints += int(input("Enter pints collected: "))

(you should check to make sure not zero, and isdigit(), but that wasn't part of your assignment))

the counter serves no purpose, so get rid of it.
The 7 iterations is taken care of by the loop.

when the loop finished, divide pints by 7
and return that value,
this can be done in one step.

return total_pints/7

so now you don't need the calculate function