A minimum of three independent sources. We also have to watch out for circular sources--that is, if I say something and you repeat it and someone else repeats it from you, the result is a circular source: at heart, there is only one source: me.Three sources, even if they are biased?
Who is to say which sources are biased? That is why you look for independence. If all your sources are from known whackos, obviously there is a jaundiced eye applied. Having a minum three sources varies field to field. Three is the traditional minimum in my field. But having that minimum doesn't assure that your work is correct--only that you have met a base criteria.