For the purpose of my narrow minded (single purpose) post......the Suzuki RE-5 rotary is a single rotor engine.......so it "IS" a single..regardless of how many sides or cycles the engine has.

There is no single correct answer as the Rotary doesn't fit into any piston criteria.  For each rotor there is one combustion chamber, and three sides to the rotor.  If you guys want to debate it.....somebody start a Rotory War thread.