Q: Is it normal for a journal to wait for reviews from more than five referees?

I submitted my paper last November to a Springer journal in the field of computer science. The submission has passed through the usual workflow of peer review. It was under review till 1st March 2018, and then the status changed to "Reviews completed". Since then, the status has remained unchanged.  Meanwhile, the status date has changed twice. I think this is happening because the submission is receiving more reviews. When I sent an inquiry, the editorial office of the journal informed me that five reviews have been received, and the editors are waiting for more reviews to come! I have read somewhere that more than four reviews are rare. Can you explain this scenario?  I am worried since my thesis defence requires an accepted paper and this delay would certainly affect my future career. I have already informed the journal editor about the situation through the staff in the journal’s editorial office but have received no response. I would really appreciate if you have any suggestion or recommendations for such situations.