where to set maximize_feed ? limit to 10 need to increase

You don’t need to alter the code to do this, you can do it in the WordPress dashboard.

Go to PowerPress >> Settings, Advanced Mode >> Feeds >> Show the most recent and change the number to 30. Click the Save Changes button at the bottom of the page.

You might also want to do the same thing at WordPress Dashboard >> Settings >> Reading >> Syndication feeds show the most recent. Just enter 30 there as well and click the Save Changes button.