How to increase the number of keywords in the free version?

Hello,

Thank you for contacting the support.

The 500 keywords limit of the PRO version are not focus keywords, they’re for the Rank Tracker feature of Rank Math: https://rankmath.com/kb/rank-tracker/

Assuming you wish to change the limit of focus keywords, you can add this filter to the website to change the limit to 10 keywords per post/page/CPT:

/**
 * Change the Focus Keyword Limit
 */
add_filter( 'rank_math/focus_keyword/maxtags', function() {
    return 10; // Number of Focus Keywords. 
});

Here’s how you can add the filter: https://rankmath.com/kb/wordpress-hooks-actions-filters/

Hope this helps.

Thank you.