what is the maximum value of SHMMAX for a 64-bit (x86-64) Linux system?

The maximum size of a shared memory segment is limited by the size of the available user address space. On 64-bit systems, this is a theoretical 2^64bytes. So the "theoretical limit" for SHMMAX is the amount of physical RAM that you have

Oracle Support officially recommends a " maximum" for SHMMAX of "1/2 of physical RAM".