Great question — answering as the UphillTech team:
The general rule is that a hole in a floor joist can be no larger than 1/3 of the joist depth. The number of cables that can fit are dependent on the size of that hole and the size of the cables themselves.
Full answer by RMDman: https://diy.stackexchange.com/a/333548 (CC BY-SA 4.0).
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