The correct option is One assignee only. On a GitHub Free plan private repository you can assign only a single person to an issue, so the maximum is one.

This keeps assignment simple for small teams and aligns with the reduced collaboration features that come with the free plan for private work. It means you cannot distribute responsibility for a single issue across multiple assignees in that context.

Up to 3 people is not supported for a private repository on the free plan because you cannot assign multiple people to a single issue in that scenario.

Unlimited is never correct because GitHub enforces caps on issue assignees and the free plan does not remove those limits.

Up to 12 people overstates what is possible on the free plan and is not available for private repositories in this situation.

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