Many 747s have operated with all-Y seating over the years, especially those used by charter carriers.
Japan Airlines (JL) and All Nippon Airways (NH) operate high-density B747s on domestic routes in Japan although they have a premium cabin on the upper deck with larger business class-type seats sold at a surcharge. JL
's 747-400D has 568 seats and their older 747-100 domestic version has 533 seats. JL
also uses some 777-300s (non-ER) on domestic routes with 470 seats which must be the highest seating capacity on any twin-engine aircraft (figures per JL
configurations are probably very similar.
The 747-400D built for JL
/NH is unusual as it lacks the wingtip extensions and winglets of other 744s; they aren't of much benefit on short domestic routes and only add extra weight. The wingtip extensions and winglets can be added if these aircraft are later used on longhaul routes.
I believe Tokyo Haneda (HND
)-Sapporo (SPK) is the world's busiest air route in terms of passengers carried with about 40 JL
/NH high-density widebodies daily in each direction.
Re the smallest types with 2-class service, many 50-seat regional jets and similar turboprops, especially in Europe, have 2-class service but with no difference in seating, only in service standards (improved meal service mainly) and other benefits that go with business class fares (lounge access, more frequent flyer miles etc).
The smallest types with a dedicated premium cabin with larger seats and increased seat pitch are probably certain CRJ-700/900s and Embraer 170/190s. AC
is one carrier with a separate business class cabin on CRJ-705 and EMB-170/190 with 3-abreast seats vs. 4-abreast in Y.