What is this?

This datapack can help you to determine the number of players that are currently online on your Minecraft server. It might be useful for creating maps, minigames or other datapacks.

May I use this in my own creation (datapack, map, ...)?

Yes, of course! That's what it was made for!
However, it would be nice if you included me in the "Additional credit" section or the description of your creation when uploading to PlanetMinecraft.