Hi Alexis,
I would recommend looking into the Attribute Assistant for ArcGIS Desktop. This will enable you to develop a rule that you create a unique identifier field and every time you add an item it add 1 or whatever increment you want to apply to the unique identifier. Here at my City, we use it for our Addressing and ever address has a Site Address ID which is a unique value like SID-(insert sequence number here). They have excellent documentation here.
As a note, this functionality will be known as attribute rules in ArcGIS Pro. They mentioned that at the UC this functionality should be coming out soon and would be platform wide, not just in ArcMap.
Greg Mattis, GISP
GIS Analyst
City of Visalia