When you're assigning keywords to landing pages, this process should be dictated by the keywords you chose within the platform.
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If you chose X keywords you want to SEO, then you need to make sure you have a sufficient number of pages to 'house' these.
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​What if you have left over pages of your website - should you add keywords to these?

Only if there are keywords that are relevant for these pages!
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You don't have to add keywords to every page of your site, that's not needed for good SEO.
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Hope this helps!