The range's minimum value is the current setting of IPCMSGNIDS or IPCSHMSPAGES, and the maximum is based on the following formula:

MIN(20000,MAX(4096,3*initial value))

The initial value is the value that was specified during BPXPRMxx initialization. If you want to use a value greater than the current maximum (as calculated by the formula) but lower than the initial maximum (20000), you will have to change the value in BPXPRMxx and re-IPL.