What ammendment to the US constitution by it's nature, applies to the fewest people?

regarding Presidential tenure, applies only to the President. However, it may be argued that it applies to any living ex-President as well.

The 25th, regarding Presidential / Vice-Presidential Vacancy applies to two people (though others in the succession line are affected).

It may be argued that the 23rd, regarding electors for D.C., applies only to their three electors.

Then there's the 27th that applies only to congress. Good show, coldfuse. (Now let someone else have a chance!)