I don't think there is a minimum threshold. Having said that, you may want to have a certain population to support the buildings cards available on the game. For example, you can purchase baisc homes, apartments and farms to start with and also buy a couple of building to let your citizens work (stadium, shopping mall).
To have these buildings you may need at least 100-200 Hive. So, 200 Hive could give you a start but that does not necessarily mean you will have a complete city setup.
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