It appears that since you filed this report, another user has added in episode titles and summaries. All the sources I'm finding for season 5 list it as 40 episodes.

I think I see what my problem was when I raised this. I looked at the total number of episodes rather than the individual ones one at a time. The episodes in S05 called Terror Tales of The Park III, The Thanksgiving Special and Skips' Story are 20 minute long single episodes but have been split into two parts. Both IMDB and TVDB have these combined but upon checking https://en.wikipedia.org/wiki/List_of_Regular_Show_episodes#Season_5_(2013-2014) and https://regularshow.fandom.com/wiki/List_of_Episodes#Season_5:_.282013-2014.29 these are split which this looks correct with a total of 40 episodes in total for that season.