There isn't a way to do this in the Generate Data GUI, but this can be done with the following steps:
1) Set it up from the App Designer. Add all of your tables and have it ready, except for the row number change.
2) Click "Apply" and close the App (but not the app designer).
3) Press CTRL+C on the app and paste it into notepad.
4) Do a global search and replace, searching for "RecordCount = 1000" and replacing with "RecordCount = 10".
5) Select All and copy to clipboard.
6) Delete the Action from the App, and paste it back in from your clipboard.