help needed on generating numeric post titles

I am creating a wordpress site where guests can share pictures. Guest creates a post where he choses an image to upload and description to add to it and gives his name (guest doesnt enter a title). How can I automatically generate a numeric title for this post upon publish where number increments with each post so there are no duplicate post titles. I also want to use this in permalink as %postname%. I have seen many websites like that but cant figure out how they do it. Is there a plugin for this or should i add something under functions.php.

I managed to add numbers to every post as a custom field but that cannot be used under permalinks.

btw I dont want to use post ID because that way numbers are not in a nice incremental sequence.