First, I would argue that a precise number would be virtually impossible to obtain. This is because there is a large grey-zone between work published in established journals and work "published" in more "questionable" sources. Obviously the way to obtain a number would be to use search services such as Web of Science, Scopus etc. or reference data bases. But, for example, Web of Science only covers works published in ISI listed journals or papers referenced by ISI listed papers and on top of that only back in time for as long as journals have submitted reference information. This means such searches will be incomplete. Hence to arrive at a number may require quite a bit of work unless one would state the limitations imposed on a search sich as limting it to Web of science.
The choice of key word(s) will also be important and it is not certain keywords are systematically applied between sources or over time.
A claim to have found "all" literature is very questionable and I would argue that when one makes such a claim one must provide a picture of the limitations of the search because there will certainly always be such limitations.