PowerApps has a limit to the number of items it can retrieve from data sources that allow delegation (including SQL and SharePoint).
To increase this limit you must go to the settings and you can change the default value of 500 up to as many as 2000. (See image below)
If you need to have more than 2000 results returned, you will have to find a different way of handling the process. This can include, among other options, getting batches of results based on an index value or Batch column and storing them in your own collection. expect that this will slow down the system for the user.
