What does the phrase '999 are playing' mean?

The phrase indicates that 999 individuals are currently engaged in playing something. Here, 999 is a numeral representing the size of a group, and 'are playing' denotes an ongoing activity in which the group is participating. Without further context, the exact activity remains unspecified.

What is the subject and action in the sentence?

The subject is the number 999, referring to a group of people. The action is 'are playing', describing that this group is currently involved in a playing activity.