Trade paperbacks or digital services like Comixology is probably what you're looking for. I'm not sure I understand this "#12 issues" thing though. The Hash appoints a single issue. 12 issues means twelve issues.
IDWs TMNT series is its own continuity. You don't have to read the old Archie or original Mirage series to understand anything. It starts proper from the first and continues as long as there will be enough readers to support it.
Sorry, I don't see that much u.s comics. I know from Dc comics, such as batman, there are mimi arcs in the series. Yeah, the the series alone keeps going but the arc it self becomes a somewhat stand alone story.
An arc is the same thing as say... The Frieza saga. That is an arc. So even if the arc is over the comic itself continues. With Batman and such its a bit different since they have been going for so long with so many different creative teams that the arcs are indeed mini stories into themselves. But for a single title like TMNT, Atomic Robo, etc that aren't old, not tied to any massive fictional company universe and is driven by a smaller team the arcs are more like on TV shows or manga serializations.