This might not be a universal answer, but it is a practical answer of how I find peer-reviewers when needed: I go to Google Scholar and search for the topic. I find which authors with Google Scholar profiles match my keyword search based on the article. Then I examine each Google Scholar profile looking at the following things:
- Total citation counts (to get an idea of total citation impact of the researcher)
- Citations on works whose titles resemble the subject I'm interested in (to get an idea of the researcher's recognized expertise on the specific topic of interest)
- How recently the author has published in the area of interest (I would be less likely to invite an expert who obviously no longer publishes in the area)
With these objective measures from Google Scholar, I of course add my subjective knowledge of the area and then prioritize whom to invite one by one. So, to answer you question more directly, I would not try to objectively decide based on a fixed number of articles or a minimum number of citations. The decision of whom to invite depends on whom I find with Google Scholar profiles and how more outstanding they are than others with whom I compare them. The decision must necessarily be subjective, though aided with these objective citation criteria.
Of course, some invited reviewers would decline or not respond, but then I move to the next expert in my priority invitation list until I receive enough positive responses.
Following this methodology, I have been able to recruit highly expert peer-reviewers whom I did not know personnally.