Hello, this is Will Hines, I'm the Associate Academic Supervisor which is sort of like the principal. I'll try to answer this, though Erik Tanouye (Director of Student Affairs) might chime in since he actually schedules the classes.
Basically, there's a huge demand for Improv 401s and they sell out very quickly. But we do try to add as many as we can, you just have to watch this message board on Sunday and Tuesday nights to see if any are going to be posted the next day.
We added 3-5 of them about 3 weeks ago for example. I know this because I am teaching 3 of them! They do get added, they just get grabbed right away.
Some of this is because students are often asked to re-take 401 if they apply to Advanced Classes and are not accepted. To help reduce demand among people who have already taken Improv 401, we just started offering 41x classes --- these are 4 week classes for people who have already taken Improv 401. There's a thread on the 41x classes here:viewtopic.php?f=10&t=28160
Basically in regards to Improv 401: they sell out fast but we do offer them, and we add them as fast as we can. You've got to check this message board to know when to grab them.