How many keywords?

Recently Google has created the SEO Starter Guide that should be able to give you the answers you need. It has proven helpful to go to the source for answers.

I would say around twenty is a safe number.

In meta keywords you can have 20 keywords , the keyword should not be repeative not more than 3 times , you can use atleast three primary keywords which has less competative and get indexed well by search engines.

There is no limit. You can target as many as you want.

Better you concentrate on main keywords in your title tag following that use some keywords on keyword tag. Dont stuff with the keywords. then SE may think it as a spam.

You should concentrate only Main meta tag that is TITLE tag. You should stuff your primary keywords in your title meta tag. You can also stuff keywords in your description meta tag also that will include the plus point regarding this.

Targeting how many keywords may depend on how long your site doing search engine optimization and how competitive your niche is... If your site is just starting doing seo, I think it would be better to focus on targeting on less number of keywords/keyphrases. Then little by little, when you see some progress on their rankings, then that's the time you can target more keywords/keyphrases.

If your site has too much competitive niche, it would be better to focus targeting on long tail keywords (3-4 keywords). In this way, it will be not hard for you to compete with those sites that are already well-established and have good rankings in serp for competitive 1 or 2 keyword.

funny thing its sometime you get better results if you simply leave google to chose for you ... I made recent a test and my surprise was to see I got 1-2 position with a keyword what I never tought about it . Most funny its I had under my blog one script who randomly pickup videos from youtube .. within like a month I think I got more than 1K visits only from this specific keyword ... and trust me its verry competitive market ...

Two per page max. I have optimized a site with 15K pages that ranks #1 for about 100 phrases and 1st page for another 300+. Deep pages can rank well if their optimized and linked properly.