According to the game rules, the more mines of the same kind you have in your game, the more managers you will need for opening a new one (the maximum amount of managers necessary for opening a mine is three). Manager for a mine can be obtained while collecting resources from the mines of this kind. In case all these mines ran out of resources, and they already got closed when a manager didn't appear yet, the first constructed not opened mine won't require a manager at all.
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