Hi there,
Welcome to SeaTheme forum.
The post ID is a unique number generated by the WordPress system to help you to identify each post on a website. It can not be assigned by default. The best way is still set wit by date, you can assign date for each portfolio(screenshot).
Or install the Post Types Order plugin(https://wordpress.org/plugins/post-types-order/). You could drag the items to change their order in the single portfolio list admin page. The grid will change order also.
Regards!
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