It is much easier to target 2 keyword per page.

I suggest you can target for the first three keywords placed in the title tag.

First of all you should go only first three keywords.
you can include one-word strings in your keywords list but if they are not backed up by more expressions; do not dream of high ratings. For instance, if you have a site about dogs, “dog” is a mandatory keyword but if you do not optimize for more words, like “dog owners”, “dog breeds”, “dog food”, or even “canine”, success is unlikely, especially for such a popular keyword. The examples given here are by no means the ultimate truth about how to optimize a dog site but they are good enough to show that you need to think broad when choosing the keywords.

Sure you can. As long as the page makes sense you can do that. If the whole page is relevant you can really build keywords for like 6-10 I would say. No more probably.

use 3-5 words/phrases. and it must be natural