In Crelate, you'll have the ability to auto number opportunities by navigating to Settings>>Core Records>>Jobs & Opportunities
When this is enabled, all opportunities created will automatically be assigned a unique ID number. Each new ID numbers is incremented by one from the last ID number. You may also alter the 'starting' number by "Resetting Next Number."
After making this select, select 'yes' to then choose which number you'd like to start with moving forward (screen shot #2).