Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.

Cards with the ability word "descend N" have abilities that care if you have at least N permanent cards in your graveyard.

For Starving Revenant's enters-the-battlefield ability, you put cards into your graveyard for surveil (if any) before drawing cards. If there are eight or more permanent cards in your graveyard by the time you draw cards, those draws will cause the descend 8 ability to trigger.

Except in some unusual cases, the cards you put on top of your library while surveilling will be the cards you draw.

When you surveil, you may put all the cards you look at back on top of your library, you may put all of those cards into your graveyard, or you may put some of those cards on top and the rest of them into your graveyard.