I was wondering when is the mark given to the players. All the marks grant major boons and I read that it is received when having 20 advances towards single alignment, and having 5 more towards it than any else. sounds pretty easy to me, and also more of a benefits for getting benefits.

Am I missing something or is it simply just given to players that easily?

You don't get the Mark automatically on becoming aligned to a God - you have to manage to roll the Mark gift as one of the Rewards of the Dark Gods (starting on p299), so you have to hit the Corruption milepoint, already be or become aligned to a God at that milepoint, and then pass an Infamy Test, and then you have to manage to roll the Mark.

So yea, it's not nearly as easy as you thought for characters to get the Mark of their god.

Actually, no, he is correct. As per pg 78, having 20 advances for a god, and 5 more for them than any other, is indeed enough to score you the mark of a god. Or you could roll it on the god-specific tables, possibly allowing you access to it earlier at the cost of another possible mutation.

So yes, there's a "surefire" way of gaining a mark. That said, 20 god-specific advances is a fair chunk of xp for most characters, and many might not find 20 advances they actually consider worthwhile the xp.

Ahh, misread the 20 advances bit - yea, that bit is correct, but otherwise (to get it earlier) you need to manage to roll it on the Rewards table.

The problem here is indeed the sheer number of advancements. There are a few alignments which are rather easy: Nurgle (Sound Constitution) and non-psyker Tzeentch (Forbidden Lore) - psyker Tzeentch is not quite as simple because you'll be tempted to drop some XP on psy rating and non-Tzeentch psy powers.
On the other hand side, certain combinations make it pretty hard to get marked - try and go for Slaanesh when you start as an Apostate and are uninterested in acrobatics and combat.