What is general rule regarding the number of reviewers? Date under manuscript status "under review" has now changed for third time within two months. What could this implicate?
It is very common to use two reviewers although other numbers occur as well. The process of getting reviewers to accept to do a review can be a long and hard process. In your case the delay may well be the result of an editor receiving negative answers to requests which means the editor will contact further persons. whether or not the date change in the manuscript handling system reflects this is hard to say without knowing what system is used and how it may be set up for the specific journal. My bet would, however, be that you see the editors multiple attempts to find reviewers to accept. At some point in the process, you can e-mail the editor to ask about the progress of your manuscript but you need to assess when the time is becoming too long and this will differ between disciplines and journals within that discipline. As an editor, it can be very annoying when people start sending such mails prematurely when one is in the middle of trying to line up good reviewers. But, as stated, it is a judgement call.
In my field (biology) most of journals use two reviewers, but the third reviewer is normally called if the first two significantly disagree.
The number of reviewers can vary. In my experience, it is normal for a paper to be by two anonymous referees (in double blind review, also blind to the authors) and the final decision will be made by an editor or associate chair who, generally, is not blind to the identities of the authors of the reviewers.
In situations where this is significant disagreement, additional reviewers can be added.
This can even vary within journals. For example, PLOSONE assigns an editor who can reach out to as many reviewers as they like or accept papers outright. Last I checked, the median number of external reviews at that journal was 2.