According to Maximum SHMMAX values for Linux x86 and x86-64 [ID 567506.1]. Oracle Global Customer Support officially recommends a " maximum" for SHMMAX of:
32-bit servers - 3 gigabytes: Note: Oracle notes that you cannot set SHMMAX to exactly 4 gig because setting SHMMAX to 4GB exactly will give you 0 bytes as max, as this value is interpreted as a 32-bit number and it wraps around.)
64-bit servers - Half the RAM - "1/2 of physical RAM" on the server
For example, if your server has 128 gigabytes of RAM, you can set SHMMAX to 64 gigabytes.
However, the maximum size of a shared memory segment is limited by the size of the available user address space. On 32-bit systems, this is two to the 32 power (2**32) pr about 4 gig. The "theoretical limit" for SHMMAX is the amount of physical RAM that you have on the server.
You can use your max SGA size (sga_target or memory_target variables) to determine the optimal setting for SHMMAX
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