It means on realm X, you can have only 1 Deathknight. On realm Y, you can have only 1 Deathknight. But technically, once you have a character to unlock this Hero Class, you could have 49 Deathknights, they would just have to all be on separate realms. The reason it says "per realm, per account" is if you have two accounts, you could have two Deathknights on realm X, one would just have to be on the 1st account and another on the 2nd. (Source)

　　你可以用一个帐号获得49个死亡骑士

　　在X服务器，你只可以拥有1个DK。在Y服务器，你也只能拥有1个DK。但是从技术上来说，当你有一个角色解锁了这个英雄职业以后，你可以拥有49个DK，他们将都会处在单独的服务器里。这个原因就是“每服务器，每个帐号”（只能拥有1个DK），它是指如果你拥有2个帐号，你可以在服务器X里拥有2个DK，其中一个在第一个帐号里，而另外那个在第2个帐号里。