How to match once per file in grep?
So, using grep, you just need the option -l, --files-with-matches.
All those answers about find, awk or shell scripts are away from the question.
I think you can just do something like
grep -ri -m1 --include '*.coffee' 're' . | head -n 2
to e.g. pick the first match from each file, and pick at most two matches total.
Note that this requires your grep to treat -m as a per-file match limit; GNU grep does do this, but BSD grep apparently treats it as a global match limit.