Use forms events to trigger generating number. Store it in DDS (for generating next) and session (for keeping track of current users ref nr) for persistence. 

Do you mean when editor creates form in epi or on submit for user? 

Hi,

I do not think had someone done something as your requirement.

But with Episerver Form as a kind of ContentType (named FormContainerBlock) then you can inherit and add more properties, for example a RefNumber property with type of string, and then in the CreatingContent event you can assign a genereated value to the property, this RefNumber property should be decorated with Editable(false) attribute so that cannot be updated through the UI.

EPiserver Form also exposes somes events (http://world.episerver.com/add-ons/episerver-forms/handling-events-for-episerver-forms/), at appropriate time you can get the RefNumber's value through the passed Event argument.