It’s not completely clear to me what you mean by “isolate the number of cards to one”. Besides, 2.15 is starting to become seriously outdated. You didn’t get an update until now because most package managers (including apt, the one used by Ubuntu so far) have failed packaging Anki since 2.1.16, so they were stuck to that version. I would recommend updating it, either by directly downloading Anki from apps.ankiweb.net or by installing a package manager that keeps up the pace. See a post I did about that.