I have a requirement, where: -
Can you please help me in achieving this.
There isn't a standard mechanism to achieve this.
You could investigate using iCall to monitor for pool member changes in the relevant pool, and modify the pool config when these changes are detected. This will cause changes to the config that require syncing (or will create additional auto-sync traffic).
An alternative approach would be to set up an iCall script on a timer, checking the number of members in the pool, and adding extra members from the standby pool to keep the number of members in the primary pool at a constant.
It's a complex requirements, and there are not any easy solutions.