free-m doesnt match with ps aux count

On Linux, cached memory is still available for use. So when looking at the output of free -m the “available” column is likely what you want to be looking at rather than the “free” one.

The output of ps can also be a bit misleading. This blog post covering memory usage on Linux has a good explanation:

Why ps is “wrong”

Depending on how you look at it, ps is not reporting the real memory usage of processes. What it is really doing is showing how much real memory each process would take up if it were the only process running. Of course, a typical Linux machine has several dozen processes running at any given time, which means that the VSZ and RSS numbers reported by ps are almost definitely “wrong”. In order to understand why, it is necessary to learn how Linux handles shared libraries in programs.