Initial Sync Count

Can someone help me to increase the Initial Sync Count when setting up an email to 5000 from 500. Kindly guide me. The maximum is 500. I tried to change it via Customize form, but here i got a message :You can’t set ‘Options’ for field Initial Sync Count

Kindly sort it out. I am able to setup it after a long time. The only trouble I am facing at the moment is mentioned.

Is there no one who can guide me here. I need a bit of guidance and it’s really appreciated. Thanks