_ untold number of people perished while attempting to cross Death Valley.
The correct answer is "an". But I have that feeling that "the untold number" = "the specific number, i.e. 12345 that was not told". So the article should or could be "the".
If there is any difference, tell me the British version of this.
An untold number is not, as you think, a number which has not been reported. Tell in this phrase retains its old meaning of count, and untold means, literally, countless. An untold number is therefore an undetermined number, one which is by definition indefinite.