Change default comment fetch count from 20 to 10 for consistency

[AKSHAT2802]

This pull request updates the default value of num (number of comments to fetch per request) in the commentsBox.get() method from 20 to 10.
This ensures that:

• The initial AJAX call (triggered by the comment meta box)
• All subsequent “Show more comments” calls fetch the same number of comments (10 by default), resulting in a consistent UX.

This change avoids unexpected jumps in the number of comments loaded on first and subsequent loads.
Trac ticket: https://core.trac.wordpress.org/ticket/53310

[github-actions]

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