You can add multiple keyphrases, just don't overwork yourself.
We have an internal rule at INK, we don't publish anything under 91%.
You can use as many key phrases as you like, but if they are not relevant to one another or the content you have written, it will harm your chances of ranking for your chosen topic.
You can add more key phrases as long as they are related and relevant to your chosen topic. But, at a certain point, it becomes irrelevant.
The main reason behind this is because Google often employs a tactic of semantic relevance for its search algorithms. For example, if a page is ranking for the term "cookie recipe", it is also likely to rank for a term like "cookie baking tips".
This means that you are better off ranking for one or two high priority keywords rather than trying to spread out your bets.
Another side of this is topical completeness, something which INK can help you with.
If you want to rank for a key phrase like "cookie recipes" but your article is entirely about airplane maintenance, it's not going to rank for your chosen term.
To show you how relevant your content is to your chosen key phrase, INK SEO Editor has a topical completeness metric.
You can find this metric in the Relevant Topics Task Card and it's one of the most important tools in the INK arsenal.
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