How many members can a tribe acquire before it's no longer a tribe?

Whatever that number is, it's the number of members required to form a metatribe.

A metatribe is not a tribe but a source of extremely reductive characterisations used to infer one's propensity for loyalty.

Why is this distinction relevant?

A metatribe mimics the benefits of tribal membership (eg, community, shared resources, meaning, protection, etc.), but it's an artificial substitute/simulation and man's adaptation to technological change.

It doesn't impart meaning, just reductive characterisations; it doesn't evaluate you, it evaluates your immutable characteristics.

Stripping its subjects down to their immutable parts and making humanity an object for consumption is its modus operandi.