It's the most populated server currently and you'll find players in just about every zone. With DoF about to release more will join or return.
It depends which TLE server you are referring to. Some of them are dead and some are not, which is why you see the people above arguing with each other about it.
Bottom line: Varsoon is a PvE server with a high, active population. Tarinax is a PvP server, somewhat active with those folks (but often you'll see them coming into the Varsoon channel on Discord, pleading with people to create characters on Tarinax, take that for what you will). Kaladim and Kael Drakel are, for all intents and purposes, dead.
Like ALL TLEs, and even some servers close to new content, people take a break and go outside for a change. Once the new release comes out (Desert of Flames in this instance) in a week, or two, it will be very populated again. (Varsoon TLE anyways)
One other side note. The active player count on Steam is absolutely meaningless when it comes to EQ2. An exceedingly small number of players (I'm guessing less than 5%) launch the game through Steam; most just use the launcher directly and have for long before Steam was even a thing.