When you create new my_part part and save it for the first time, you will get my_part.prt.1 file.
When you add some features to my_part part and save it for the second time, you will get my_part.prt.2 file.
... and so on
If you finish Creo session, start the new one and open my_part.prt, then Creo opens my_part.prt with the highest numeric extension.
If you are sure that you do not need files with lower numeric extensions, you can delete them. BE CAREFULL AND DO NOT REMOVE FILE WITH HIGHEST NUMERIC EXTENSION !