Per is more precise, but a is more common.

per

: with respect to every member of a specified group : for each

a

—used as a function word before singular nouns when the referent is unspecified and before number collectives and some numbers

They are both right, but a is more common.

per (preposition)

: with respect to every member of a specified group : for each

a (preposition)

: in, to, or for each