We generally consider T're Asar as one book (as is evident from Bava Basra 14:2). Are there any ways in which we officially treat it as twelve?
(Possible ways I can think of that we might are: having breaks between the twelve sections of the sort that are normally between s'farim, when in a k'laf (scroll); a rule that applies only to complete s'farim applying also to a scroll containing only one of the twelve (tum'a on hands perhaps??); a rule that applies to studying a complete sefer applying also to studying only one of the twelve (siyum b'choros perhaps??).)