I've a feeling I've been here before in another dictée ! I thought that it was the number (singular)[of vowels] that was present. I can't get my head round why "présentes" agrees with "de voyelles". If "de voyelles" weren't there, it would read, "..le nombre présent au tirage." "The number" is still something singular, however many things it might be encompassing, surely ?
What am I missing here?