I believe that Slobodan Milosevic, ex-president of Yugoslavia, is the first and only head of state to be tried by an international court for war crimes. His trial is still ongoing. I don’t know if other heads of state have been tried for war crimes by a single country, however.
Admiral Dönitz was German head of state after Hitler’s suicide, and he was convicted by the international tribunal at Nuremberg (for crimes against peace in his previous role as commander of the U-boat force, though).
I know General Hideki Tojo (the Japanese prime minister during the war), Marshal Henri Philippe Petain, and Pierre Laval (Head of State and Prime Minister of Viche France, respectavely) were all tried.
Has it really been just that small handful?