I upload my stl file on MakePrintable online stl file repair software and I got some details about my file. You can see on the attached picture. What I need is count the number of shell objects, but I have no idea how can I count just from the facets in stl file. If anyone could help me, please leave me an answer.

I think what that is telling you is there are 31 separate parts or shells making up the part. So like if you jut had a basic cube then it should be just one shell.

Ideally you would want it to be one shell.