I think both could be true. Consider T to be the likelihood an individual will intervene, N the number of individuals, and P the probability of an intervention.

I think both could be true. Consider T to be the likelihood an individual will intervene, N the number of individuals, and P the probability of an intervention. N, T = 1, 0.5 then P = 0.5 N, T = 10, 0.2 then P = (1-.8^10) = 0.9 Basically, the likelihood of an average individual to intervene may decline as there are more people, but the likelihood that at least one individual does intervene increases as crowd size does.