Hi Xu Yan,
From the documentation and SAP Note 48284 - System can no longer create spool requests
rspo/spool_id/max_number : Maximum spool number in system
The parameter determines the maximum possible spool number in the system. The customer cannot change this value. The only value permitted is 99000, the same size as rspo/spool_id/loopbreak.
You can raise the upper limit for spool requests. As of Release 4.0, you can set the upper limit to anywhere between 2 and 31 numbers (previously 99,000). However, we recommend that you do not set the interval higher than 999,999 because the human user finds higher numbers difficult to process.
The size of the interval also determines the maximum number of spool requests that can exist in the system.
Both the parameters are different. I can say that rspo/spool_id/max_number is not of use if we set the number range object SPO_NUM to value higher than 99,000 ( upper cap for this parameter rspo/spool_id/max_number). But it is not recommended to have values higher than 999,999 as per SAP note 48284.
Practical suggestion is keep the value to SAP recommended one and regularly schedule housekeeping jobs
SAP_REORG_JOBS
SAP_REORG_SPOOL
Hope this helps.
Regards,
Deepak Kori