The curve will be particularly complex. When there's no one. No curve. Very few, then it's worth to collaborate even when they backstab you. Specially if the environment is super-dangerous. You want to create a story according to which they didn't betray you at all. Anything, as long as the mutual knowledge is still on the friends side. Bigger numbers: Something close to Dunbar number would probably be where you most need to signal trustworthiness, and from then on, the more there are, the lowest is the cost of free-riding.