100% In practice I guess the correct answer is less than that since the icons for the existing pre-made ones can be redone (and maybe could be different from theme to theme, not sure), and a future update could change the system, but in general we can use or not use as much or as little as we want of it.
As for what to use or not, I don't think I should make the decision, but I want to add my opinion: I personally like the possibility to show that I agree or appreciate something without having to take up unnecessary space or go off-topic. I do think that it should be limited to positive reactions though, for negative reactions to have a positive effect (if all you get is "I dislike this" you are unable to change or explain whatever it was that caused the reaction, so all the reaction would lead to would be to make the receiver sad) they need to be expressed in greater detail (preferably phrased more politely than "I dislike this" as well for that matter), so my preference is to keep it to positive or at the most neutral reactions. All reactions are neutral or positive in terms of how they affect the "reputation" (i.e. the little number next to the plus symbol underneath your user group and avatar), but it's of course up for debate how they affect the one receiving them, so I wouldn't say it has to be the way things are now.