Has it occurred to anyone that perhaps Sofia is a Deputy Chief in Los Angeles, not Las Vegas? With this new information about episode 21, that would make more sense. Don't the Feds get involved only if a criminal crosses state lines? Some key plot details are still unknown.
According to the plot description for episode 20, Haskell is supposedly living next to a farmer with a barn full of body parts. We know that there is a 2-episode arc involving Haskell which is not the season finale. Carol M. hinted about Louise Lombard possibly returning for an additional episode (the season finale). If episode 21 is the one she's appearing in, then it's not the season finale. If episode 21 airs on May 5, there's still 3 more weeks of May sweeps left. CSI has gone into the 3rd week of May before.
I'm 99.999999% certain there's going to be 23 episodes.