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Are the servers still active?
About how many are on at a time?
Does it still require a sub?
While it seems that the game is still running it is not advertised anywhere on the Sony webpage an more, it still is a sub game but i can not tell you how many are on at any time as i do not play it any more. I did play it before the release of PS2 and at that time it had a very small but active player base. So by now i think you will be OK during your regions prime time but outside of that i think it is pretty dead.
digging around here might help (PS1 official forum)
or here (Planetside fan community forum)
To get a feel for if the game still have a viable population.
This have been a good conversation