As of 20 May 2018, 116 episodes have aired over 19 series. Episodes 1 to 100 were originally released on DVD as 25 "sets," which are now discontinued, and have been re-released as "series" 1 to 16 in redesigned packages.

Answer this question based on the passage: how many series are there in midsomer murders? Okie... think carefully first, then make a decision:
The important part for answering this question is: As of 20 May 2018, 116 episodes have aired over 19 series. So, the answer is 19.