So each episode seems to have one of their already known songs in it. They only have about 10 songs. What are they going to do when they run out of their music to use? They've been around for a while and haven't really done a whole lot.
The birthday song at the beginning of last week's episode was an entirely new song. Just because they only have a limited number of songs posted on YouTube doesn't mean they don't have more to record. Kate writes and performs songs she has written for playing solo, I think Riki does the same thing. So I don't think this show is only limited to how many songs they have currently released as Gaefunkel and Oates. IFC also does very short "seasons" of shows, on IFC eight episodes is a whole season. Portlandia only gets ten episodes every season.