Internal links are links on a page on your site that go to another page on your site. Inbound links are other websites that link to your content. Both are valuable for SEO.
Unfortunately, I can't tell you an exact number that is the "right" number of keywords on a page, mostly because that's the wrong way to think about keyword optimization.
There's no keyword density you should be aiming for -- in fact, using a keyword too many times can result in penalization due to "keyword stuffing." Just keep the reader in mind, and only use keywords when you need to. You'll find enough natural opportunities to include keywords that you won't even have to worry about reaching an arbitrary number.
Organic results are the results that appear in search engines, for free, based on an algorithm. Paid -- or inorganic -- search results appear at the top or side of a page. These are the links that advertisers pay to appear on different search engines.
SEO stands for search engine optimization. It refers to techniques that help your website become more visible in organic search results for the people who are looking for your brand, product, or service via search engines like Google, Bing, and Yahoo.