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There is no magic number. As a loose sanity check, a primary keyword landing somewhere around 0.5%-2.5% usually means you have written naturally about the topic. Treat that band as a smoke alarm, not a target: if you are well above it you are probably repeating a phrase for machines rather than readers. Write for the reader first, then glance at density to catch accidental over-repetition.

Yes — but only at the high end. Cramming a phrase in repeatedly (keyword stuffing) is a classic over-optimization signal and can trigger a spam or quality demotion. Modern search engines read meaning, synonyms and context, not raw repetition, so stuffing gains you nothing and risks a penalty. Low density is almost never the problem; unnatural, forced density is.

Word count is not a direct ranking factor and there is no minimum length that earns rankings. What helps is fully covering the topic and answering the searcher's question — sometimes that takes 300 words, sometimes 2,000. Padding a thin page to hit a length target usually makes it worse. Use the count to check your copy is complete, not to chase a number.

They are terms semantically related to your topic — the words that naturally appear when someone genuinely covers the subject. (LSI is an old information-retrieval term search engines do not literally use, but the idea of related vocabulary is real.) The takeaway is to write for topical coverage, not for a density figure: mention the related concepts, synonyms and subtopics a reader expects, and the relevant phrasing takes care of itself.