I noticed that since 2008, CBS has always given a freshman show with a full season an extra episode or 2 and that show always gets renewed which has led me to believe 'extra episode= renewal'.
(The 2007-08 season didn't have any show with 22+episodes because of the Writers Guild Strike)
To prove my statement, here is data from 2008 to 2015:
2008-09: The Mentalist(23)
2009-10: The Good Wife(23)
2010-11: Mike & Molly(24)
2011-12: Person of Interest(23) and 2 Broke Girls(24)
2012-13: Elementary(24)
2013-14: The Millers(23)
2014-15: NCIS: New Orleans(23)
My point here is that currently only 2 shows with full seasons, Limitless and Life in Pieces. and knowing CBS, at least one of them will get an extra episode, guaranteeing it's renewal.
Supergirl won't get an extra episode because it has 20 episodes and due to it's long production time, cannot make more episodes in time by May.
Thanks for reading, and if you have any thoughts on this, feel free to comment down below.