There are standards for how many independent inputs are needed based on the criticality of the system. It's not just a guess as to how many are sufficient. That's why the categorization of MCAS correctly ('catastrophic' vs. 'hazardous' is important)
In designing airframes, there's actually guidance along these lines to remove the ambiguity of how many sensors are necessary. You can use calculations to define what level of reliability is sufficient. As an example to this point, there are standards like IEC 61508 that outline the procedure for doing such calculations. Many organizations also create their own standards (e.g., five redundant sensors for mission critical systems, self-diagnostic sensors for safety critical systems etc.) It shouldn't be guesswork. It's a risk-based decision, not a subjective guess as to whether two or "a hundred" sensors are necessary.