What this referenced in the Star Trek: Picard or the Countdown comic? Or can we tell from his 2485 uniform?
I believe he had four pips inside a rectangular bar. That makes him an admiral which matches all the references I've seen; e.g. Admiral Jean-Luc Picard, Retired¹.
First image is a screenshot from the flashback scene in ST-PIC S01E03 'The End is the Beginning' immediately following Starfleet's acceptance of Admiral Picard's resignation with his rank insignia showing the four-pips-in-a-bar of a flag officer admiral. Second image is an image detailing Starfleet commissioned officer ranks and associated insignias. Click for larger image(s).
As an Admiral, he was one rank below Fleet Admiral (5 pips inside bar), and above Vice Admiral (3 pips inside bar), Rear Admiral Upper Half (2 pips inside bar) and Rear Admiral Lower Half (1 pip inside bar)².
These pips parallel the stars accorded the admiralty ranks within the US Navy.
FWIW, the rectangular bar surrounding admiralty rank in Starfleet always reminded me of the rectangle with rounded corners encompassing an Egyption pharaoh's cartouche. The cartouche's surrounding border has been interpreted by modern scholars as a means to protect the pharaoh from evil spirits in life and after death.
¹ Memory Alpha lists Jean-Luc Picard as one of the individuals who attained the flag officer grade of admiral in Starfleet. Inclusion in these Starfleet lists is typically made only at the highest rank that an individual has attained; e.g. Picard is not listed in all the ranks he attained up to flag officer admiral, only flag officer admiral.
² Memory Alpha has done a pretty good job of providing information pages for Starfleet ranks and flag officers. There are individual information pages for the Admiralty (Fleet Admiral, Admiral, Vice Admiral and Rear Admiral (Upper and Lower) in addition to others.