The typical workflow is that you fork the peps repo and edit your file giving it number 9999. When you submit it as a PR, a PEP editor (probably me :-) will assign you a number and then it's a simple commit to change the number in the header and rename the file. This way we don't have to keep a database of assigned PEP numbers -- conflicts are resolved by repeating the process (though I don''t recall this has ever happened).