Yup, definitely possible.

Thinking in "sets of keywords" is the wrong way to look at it. SEO optimized content is about writing in a natural, human way, and almost unconsciously using keywords within the content (readable, valuable content is crucial). That way, your keywords don't read like keywords, and you also seed your article with potential "long-tail" keywords, which is where the real money's at.

IMO, learn the basics of how Google's algorithm works before trying to rank a website. It's simple, and will save you the heartache of building up rankings for months, only to get slapped by a Google update.

Here's a good guide for it:

SEO: The Free Beginner?s Guide From Moz

Yes, it is possible.

One blog post has main keyword and a lot of long tail keywords. Almost my traffic come from long tail keywords.

My advise is to focus on few keywords and mix them, Focusing in one keyword will trigger that bot and using many keywords will not be very effective or easy.

Each blog post will rank for multiple keywords you didn't even attempt to rank for or target within your post.

Of course it is possible. It is best to target long tail keyword (4 to 6 words). And consequently can win a shorter keywords...

yes your domain can rank for multiple keywords there is no limit.

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