*[Per][1]* is more precise, but *[a][2]* 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 <a man overboard> and before number collectives and some
> numbers <a dozen>


  [1]: http://www.merriam-webster.com/dictionary/per
  [2]: http://www.merriam-webster.com/dictionary/a