Don't think so, not a data based fact one.
Ignore the nay Sayers etc and go by what you see. I see towns with pop at writ drop offs / pledges. I see people in Vivec, i queue 1 min for a Battleground evening. 2-3mins tops as a healer for a dungeon. Pops enough for all i need.
PvP if you take a mule to Vivec and hit AP you can see how many active that campaign as you will be in last place.
Don't think so, not a data based fact one.
On PS4 you can look at the percentage of players that have earned trophies for common and not so common game achievements. That is about it as far as I know, and won’t give you any idea how many are currently playing the game.