Anders from the Shopify Social Care team here. Thanks for reaching out.
While there isn't a way in the theme editor to add more than the 5 product rows in the Featured Collection section of Debut, a workaround would be to add a second featured collection section and delete the top text. By doing this, the two featured collection will actually blend quite nicely - as if you were adding more rows. I've provided a screenshot of what the section should look like below:
If you run into any troubles with this, let me know!