You need to have all the I20s. That means any I20s issued since you were a student. I don't think your college keep a backup. They just print and give it to you. So, your best bet is to contact USCIS and ask them instead. They must have some way out of it.  If you miss one I20, then your OPT is rejected. It happened to my roommate. So, you better be cautious and apply once you are 100% sure about the documents.