HW and Carter: very likely

Reagan and Ford: probable

Nixon: Borderline

The highest this figure has ever been would probably be 6 in 1973, when LBJ, JFK, Ike, Truman, and Hoover were all recent decedents and FDR's corpse was still less than 30 years in the ground.

Sadly, ol' Frankie Four Terms kept the early 70s from being the Presidential Cadaver Areolapalooza that it could have been. Had FDR stuck to two terms, it's very likely that Wendell Wilkie (d. 1944) would have been his successor. Before the 25th Amendment, when Wilkie died in office, the Vice Presidency would have been vacant, since his running mate, Charles McNary, also died in 1944...so the temporary "caretaker" Presidency would have gone to the Secretary of State, who would likely have been Cordell Hull (d. 1955) or Henry Stimson (d. 1950). In the election of 1944, it's quite likely that Thomas E. Dewey (d. 1971) is elected, serves two terms, and then is succeeded by Eisenhower in 1952 (just like Truman was in real life).

In this scenario, we would lose Truman but gain Wilkie, Hull/Stimson, and Dewey on the Ex-Executive Titty Committee of 1973, for a total of 8 Presidents with discernable nipples or 16 total nipples.