Only deceased U.S. Presidents. Gerald Ford gets one in July. Clinton, Carter and the Bushes each get one after their time on Earth is done. It's tradition, so no kvelling about the appropriateness of honoring any certain president.
Few other national office-holders are according a commemoration. The most under-represented group in this regard is U.S. Vice presidents.
By my count, there are 31 deceased people who served as Vice President without later becoming President. Of these, only one has been singled out for stampage:
- Hubert H. Humphrey (under LBJ). Humphrey's stamp wasn't even a commemorative; it was a regular issue 52-cent stamp for heavier letters.
- Elbridge Gerry, who served under James Madison, was depicted, but not named, as he was one of the signatories to the Declaration of Independence and was thus shown in the John Trumbull painting, itself printed on a few stamps, …
- …which was not of the actual signing of the Declaration, but its presentation by the drafting committee to the Contintal Congress; Trumbull just stuck in as many of the signers as he could. But I digress…
Many other historical people may be long overdue. I only count four Supreme Court justices, although several Supreme Court rulings have been commemorated. Only eight Cabinet Secretaries and nine U.S. Senators. There's still a lot of history to be explored.