Note this question is asking about nodes failing 'simultaneously', not nodes failing one after the other.
With six nodes and one witness, there are seven votes. To maintain quorum there needs to be four votes available (four votes is the majority of seven). This means that a minimum of three nodes plus the witness need to remain online for the cluster to function. Therefore, the maximum number of simultaneous failures is three.
Reference:
https://docs.microsoft.com/en-us/windows-server/storage/storage-spaces/understand-quorum