how much innodb_buffer_pool_size need to keep under my.ini file?
innodb_sort_buffer_size defaults to 1MB.  That is probably good enough.  However, since you have a huge amount of RAM, let's say 4MB might be a little bet better.

I assume you set innodb_buffer_pool_size to somewhere around 500G -- Setting that is the most important thing for InnoDB.  (Exactly 500 is not important.).  And innodb_buffer_pool_instances should be 16?

The above recommendations assume that all (or most) of the use of the server will be for MySQL.  And it assumes you are using InnoDB, not MyISAM.