Where is the submitted $_POST array stored after an option page submission?

You only need one callback function, as defined in your call to register_setting() (Codex ref.):

register_setting( $option_group, $option_name, $sanitize_callback );

Thus, all options are contained in an array, in a single database entry, $option_name.

Then, the callback function is passed the $input variable that holds all of the form-submitted data, the function manipulates/sanitizes those data, and returns the sanitized output:

function mytheme_sanitize_callback( $input ) {
    // sanitization functions go here
    return $sanitized_output;
}

So: one database entry, as an array of options; and one sanitization callback for form-submitted user data.

For more help, you might reference this tutorial.