I think Delta has about 130 flights a day from JFK. The airport is their primary gateway to Europe (Athens, Barcelona, Dublin, Frankfurt, Lyon, Madrid, Milan, Manchester, Rome, Paris, Stockholm, Warsaw, Istanbul, Moscow, Venice, Stuttgart, Munich, and Zurich) and probably some others I left out. They have feeder flights to places like Houston, Seattle, LAX, SAN, SFO, Boston, DCA, code-share with South African to JNB, Air France (which operates from Terminal One), PDX, ATL, FLL, and Delta Express also serves some Florida markets. Delta operates from Terminals 2 and 3, the old Pan Am worldport (T3). Terminal 2 was once the Delta/Northwest terminal, but Pan Am acquired it in the 1980's.
JFK is not considered a hub. Much of the traffic is O&D though with the feeders, there is beyond service. For instance, they fly a 767-300 from Hartford, CT to JFK, then on to Europe.