Check for plumb at first to see what kind of slope the deck has.
Most decks are sloped no more than 1" every 4'. That is 2%.
If you are installing on wood you can just drive the screws down a bit farther on one side to plumb the post perfectly.
If you are on concrete and you want perfection, use a single stainless or hot zinc galvanized washer as a shim on one side.
Ignore the green masking tape. This was a vinyl deck surface. With blocking under the deck surface, six 1/4"x3" or #14x3 screws will do the job. You can use eight if you like.
With concrete screws, one in each corner is sufficient.