1 or 2 will be plenty. A game like this is a bit of a niche thing and the kiddies won't be rolling in in droves.
Plus I don't know how many will stick around once they get a look at the game based on several factors (many of which are unknowns).
Since I'm one of those anit-censorship guys that people love to hate; I'll elaborate on this post by stating I'm not talking the censorship being a point of issue in this post.
I'm referring to things such as:
-Replay value. In Alpha each character class and faction from a story progression were 99% identical. If this is the way the full game will be, there's very little replay incentive.
-PvP. PvP guys want a fun, balanced experience. If this game has characters classes that are heavily under powered vs others, that will discourage some of the PvP folks.
-PvE. Nobody in PvE likes gank-fests or griefers. If the game is filled to the brim with this sorth of thing, people are going to get fed up and quit.
-Pay 2 win. This is pretty self-explainitory I think. This set up will ensure that some player quit playing.
-Item shop design and layout. Will items be fairly priced? Will they be 30 day rentals or permanent purchases? A good number of people aren't going to be happy about "renting" swimsuits, seal removers, etc. for 30 days at a time. Most people will want a one shot purchase for things like this.
-Grinding. Most gamers I know (myself included) aren't big fans of the mindless grind. Being stuck in a situation where grinding is the only viable choice you have to level up will frustrate and irritate a lot of people that don't enjoy doing it.
-Level progression. Will there be plenty of content to avoid content gaps? Or is this a MMO that doesn't have enough leveling content to keep things going smoothly? If a game doesn't have enough content to allow a player to level from 1 to max smoothly, there's not enough content in the game to facilitate the level range (see the above "grinding").
-Economy. If people don't have enough gold at the end of the day to use the auction house for their needs due to an overpriced market, people will get frustrated (especially those who aren't into hours of grinding just to be able to afford basic necessities).
More happy people = more happy players = need for more servers.
As stated above...a lot of unknowns.