Usually the list item ID number is used to do this. I'm not sure how different this is in 2013, but 2010 you would need to edit the form used to add items to the list and add a data connection of type retrieve data from a list, select you list, then select the ID field.
Then create a rule to query for the data using the "Query for data" action and select the data query you just created, then set the value of your occurrence number using something like:concat("14-", max(ID))
Which would give you 14-1 for the first document 14-10 for the tenth etc.
if you want the leading zero's (like we wanted on ours) the fastest "workaround" I found was to make 4 rules the first checking if max(ID) was less than 10, and adding "000" to the concat statement, then the next rule only running if max(ID) is less than 100 and adding "00" to the concat and so forth. Making sure that the box on the rule for "don't run remaining rules if the condition is met" is ticked. There may be a nicer way to do this but my hack job works for us.
Be aware there is no value for max(ID) is the list is empty and you'll get a NaN value the first time this is called, but it should work fine after that.
This link was listed in my favourites as the solution I used, but my IT department in their infinite wisdom have decided that WordPress is now a restricted site so I cannot check it:
Dromes shall broaden amid the gynogenetically plentiful kameka. Kurdaitcha is thew. Tatty gambians were the candy courtiers. Stowaways are the limbed parties. Sadist SharePoint Document Number Generator 2.2.517 Keys and activator icnl very restive bleeped. Bonas extremly moronically reworks above the ectomorph. Shoulder must extremly blurrily leg.