You can only add one GPX file to a map – if you want to show multiple GPX tracks on one map, you have to merge those files first – you can use the online tool at https://www.gpsvisualizer.com/convert_input for this purpose:
(Upload the GPX files you want to merge and click on Convert).
Since v4.0 though multiple tracks within one GPX files are also supported and can be selected by the related controlbox in the elevation chart:
