What is the recommended word limit for essay writing?
If a word limit is not set by the examining body, then what would be the average or ideal word limit for any given topic?

I am assuming you are refering to CSS Essay? In that case, it is indeed set by the examining body. It says in the 2017 question paper that the essay length should be between 2500 to 3000 words.

I don't know if that is to be strictly followed but since they set the topics themselves, I am sure they are expecting people to need that many words to get their point across. I have heard though that people have passed without being in this range. Someone said a guy passed with 800 words only but I find that hard to believe.

2500-3000 is the word limit for essay. One should try to write essay within that limit. Exceptions exist in which students have claimed that they have passed essay paper and they have written less than that limit. Those papers were exceptional papers of extraordinary students who has excellent essay writing skills. They were successful in convincing examiner in content less than that limit. I think their essays had one of the best outlines, introductions and conclusions. Not all of us possess such essay writing skills so, one should follow the instructions. Try to write 14-18 pages including outline. Do not write less but you can write more. One cannot count words. Length of outline should be 2-3 pages. Keep pages in mind. I wrote around 20 pages in 2017 and I have passed the essay paper with pretty good score. My outline was 2.5 pages long.

__________________
WA-TU-IZZU MANTASHA; WATU-ZILLU MANTASHA

Examiner checks quality rather than quantity. Number of pages depend on your writing style. For instance, I can write an essay of 2500 words excluding outline within 11 pages (both sides).