When asked how many singers they do hear, test persons may say:
more than 3, 5, 10, ...
- 3, 4, 5, 6, 7, ...
At the heart of discriminating and thus counting singers just by listening lies the ability to distinguish voices by timbres, i.e. to analyse a highly complex frequency spectrum. Experienced listeners will typically perform better than unexperienced ones. Disturbing effects of reverb and so on must be taken into consideration (and possibly be minimized or even eliminated).
Are there reported "world records" like
X was able to reliably tell if there were 3, 4, 5 up to 12 singers singing?
Is there scientific literature on this topic?
You may want to compare these questions to this seemingly more natural one:
Let n voices play n different tunes (as in a big band or any kind of orchestra) - how many voices can be distinguished (by the most experienced listeners)?