Dennis @ PerfectForms
Each instance has a unique sequential number – the instance number. After the form is published the first time, the first instance starts at instance #1. You can simply use the instance #, or use a formula to build on it.
Remember that an instance # is not created until the instance has been submitted the first time. You may need to create a behavior on when form opens that sets a field (call it hidden instance # helper)with Special – Instance Number. Then use a simple branch to look at at that field. If the field value is less than 1 – submit and re-open the same instance. If it is = or greater than 1 go to an Exit behavior object.