Technically, there is no limit to the number of keyphrases you can add to one INK file.
However, using too many keyphrases is usually not a great idea, particularly if they are not topically relevant to one another.
Let's say you're writing an article about baking cookies. Great keyphrases to use would be ones such as "cookie recipes", "cookie baking tips", or "best chocolate chip cookies".
You could add plenty more keyphrases that would be similar to those. 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 keyphrase 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 keyphrase, INK has a topical completeness metric.
You can find this metric in the Relevance Tasks bar, and it is one of the most important tools in the INK arsenal.
Ideally, you should aim for a score of 100%, but anything over 90% will suffice.
In short, you can use as many keyphrases 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.
If you are having issues with adding keyphrases to your .ink files or have any questions, feel free to send us a feedback form through the INK app.