How much resilience is sufficient: single, dual, triple, quadruple or more?
You need to understand what is the consequence of system component failure to the committed service per agreement.
It is the kind of balancing risk for optimal investment. Even if there is penalty clause for breaching the committed service level, the amount paid out might be much less than the TCO (Total Cost of Ownership) of investing a robust infrastructure and the recurring running cost.
Nevertheless, intangible loss like brand name or reputation damage need to be considered.