Fully Custom servers dont hit 500 people because they almost never have the infrastructure to handle even more than 150 and have people still be happy. Deathzot 8.6 V1 didnt really have enough ground to make even 70 active people not feel crowded to all hell, and 70 is probably being generous. V2 looks like some large map scale improvements but the infrastructure will still not likely break a soft cap of 140 players online. because that would require everything from the starting 0-200 zone to the 15k+ areas having double or triple the sqm they are now. and at the start that would feel like supermassive overkill, but its a better option than running into a wall where u have to many players and not enough space thus resulting in anger, and people leaving.
Also, Deathzot is known for having its scumbags who will chase off other people and completely make them quit, even new players who only been playing a week or two, these players Dont come back. and as a result, a slow decline in playercount also happens.
it takes allot to make a custom server have the potential to hold big online numbers, Deathzot has the features, but i feel lacks the spawns for huge online count. ( not saying its spawns are small, they are nice ) and the asshat community is what will keep it from being massive, but i still predict it will be large.