What is the command for finding the number of bit of an OS/390 or a z/OS?
While I was not able to find commands to give this information, I think below is what you're looking for:
According to this: https://en.wikipedia.org/wiki/OS/390
z/OS is OS/390 with various extensions including support for 64-bit architecture.
So if you're on a zSeries processor with z/OS, you're on 64-bit.
According to this: https://en.wikipedia.org/wiki/IBM_ESA/390
OS/390 was installed on ESA/390 computers, which were 32-bit computers, but were 31-bit addressable.