sharp and continuous drop of google indexed pages

I am experiencing a sharp and continuous fall of my pages indexed by google as seen in google console. My site is a classified ads site with more than 30K pages indexed in Oct. 2018 but now it is 8K and still dropping (see attachment). I need somebody with experience to investigate an provide solution. Please bid if you have experience no try and error please. I am available to answer questions and indication of any changes made on the site