I would like to know how can I say correctly an aggregation on a certain category, for example, let's say that I want to collect event data and display the amount of events for each user. Then, is it correct to say "I am aggregating events on a per-user basis"? Is it maybe more correct to say "I am aggregating events per-user? Can the prefix per- be used for any possible category? Am I maybe overusing it, while some other form might be preferred?
Per is a useful preposition to indicate different categories. As an alternative to per user basis, you can say on a user basis. Ngram
Per- as a prefix means throughly or intensely or completely as in perfect(per-fect), perplex(perplex) etc. So here use of per as prefix is not advisable or correct.