If your question is about commercial ops (JAROPS), any perf class A aircraft has to be operated by 2 crew, even if FAR/JAR 23. (e.g. CitationJet). A perf class B aircraft may be operated single hand. perf class B is limited to max 5700kg - so the KingAir B200 is most likely the "biggest" single pilot twin.
There are hybrids like the C500 - a true 2 crew aircraft (AFM/POH says minimum crew - 2) and the C501 - same aeroplane, different AFM/POH (okay pedants, there is an autopilot, boom mike etc) that is a single pilot aircraft. (only private ops in JAROPS country).
Under JARFCL, it is (theoretically) quite easy to seperate them: a 2 crew aeroplane requires a TYPERATING, a singlehand aircraft a CLASRATING.
The new CJ3 is somewhat in between, beeing over 5700kg, but still a class rating C525...
(No rules without exemption!)
