What answer do you expect here? There's four people referenced in the sentence. There's more implied because of Mothers, but if you're including transient dependencies, where do we stop?

It can also be 3 people, as one person can be a father and a son at the same time. If you allow non-mentioned people to be included in the attribute (i.e. the sons of the fathers are not part of the 2) it could also be 2 people, as long as they are fathers.