You can have as many hots sharing a neutral as there are differently-phased legs in the system. A split-phase system can have two hots per neutral. A 3-phase system can have 3. If you manage to find a 2-phase system somewhere, it could have 4. There is one service configuration that could allow 6 hots on one neutral, but I doubt it's ever been used in reality. It has nothing to do with whether the wires are in conduit or not. You can never upsize a neutral to allow it to be shared with multiple hots on the same phase. The only way you get to share a neutral between hot conductors is if the current in the hot conductors cancels instead of adding. Thus, no up-sizing.
I am a lawyer, but not your lawyer. And who cares anyways? We're here to talk construction. This is DIY advice, not legal advice.
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