Hi, can i sacrifice any character as a action or is it only those with some kind of rule (Forced response or any other) that can be sacrificed? and how many characters can be sacrificed?

Sacrifice cannot occur unless something triggers the sacrifice through an ability.

In other words, you cannot sacrifice a card just because you want to without the help of another card ability.

Ritual of Summoning is quite powerful because it allows you to choose a character that you control and sacrifice it at your own demand. In the case of this particular card, this happens at the speed of an action... because it is an action.

Here is a list of the 80 cards as of the date of this post that involve sacrifice

As an aside, it is interesting how often this question gets asked by people in my locality now that sac mechanics are more relevant in the game.

Just like in real life, if you want to sacrifice somebody, you have to be performing a ritual that requires them to be sacrificed in the first place. Non-ritual slaughter is just a waste, isn't it?