e-learning view The Manual here
IMPACT ON PEOPLE AND THE ENVIRONMENT
Dose

INCREASING THE RATING DEPENDING ON THE NUMBER OF PEOPLE EXPOSED

 

We now need to understand how INES takes into account the number of people exposed.

For the criteria related to stochastic effects, the minimum rating should be increased by one level if excessive doses are received by 10 or more individuals, and by two levels if the doses are received by 100 and more individuals.
For deterministic effects, a more conservative approach is taken and the rating should be increased by one level if excessive doses are received by several individuals and by two levels if the doses are received by a few tens of individuals.

The actual definitions are deliberately a little vague.

However, the Manual contains a footnote which says:

it may be considered that ‘several’ is more than 3 and ‘a few tens’ is more than 30

the basis being that 3 is approximately half an order of magnitude on a logarithmic scale