A common mistake when you start with Qlik Sense extensions is to forget about setting Initial data fetch in your extension. Typically you would include something like this in the initialProperties of your extension:
That would work, and make sure your extension gets the 500 first rows of data in the layout provided to your paint method. But sometimes you want the app developer to be able to set the number of rows fetched. In that case you can simply add the qHeight parameter to the property panel like this:
And the result is a new section in the property panel, where the app developer can set the number of rows initially fetched.