How do I get the total number of audio files that are playing in a given second?

Unfortunately, there is no expression for that.

An idea maybe is to keep track manually by adding the tags into a dictionary keys and then you can just check "Dictionary.KeyCount"

You will just need to remember when you stop an audio make sure you remove the key from the dictionary.

or

By making a loop to check all the audio tags one by one to see if it's playing, then add to variable 1 for each tag that you find playing.