how many random maximum nodes can go down but database cluster continues to alive without any issue?

It depends..
K-safe 2 ensures that Vertica will run normally if any 2 nodes fail.
Potentially, with K-safe 1 or K-safe 2, up to half the nodes -1 in the DB could fail, without causing the DB to shut down, as long as the data on each failed node is available from another active node.
Because of the extra space and I/O required for each copy in K-safe 2 configuration,
It is advised to use K-safety of 1 and define fault groups to reduce the risk of failures when two or more nodes fail.
One can also use fault groups to make Vertica be rack aware.
For more info about High Availability and Data Safety see: https://www.vertica.com/kb/KSafetyBestPractices/Content/BestPractices/KSafetyBestPractices.htm