Horror Tests with Multiple Monsters and Multiple Investigators
Yes, you will make a total of 8 horror tests. One for each monster that enters an investigator's room, and for each investigator.

Rule book (page 13), within the section covering Horror Tests.


  Whenever a monster enters (or is placed in) an investigator’s room (or vice versa), the investigator must immediately make an attribute test called a HORROR TEST. He does so by making a Willpower test modified by the monster’s horror rating (the blue number at the top of its monster token). If he fails this test, he takes one horror (see “Damage and Horror” 
  on page 24).
  
  If multiple investigators are in the room, they must each make a horror test (the keeper chooses the order).
  
  If the investigator leaves the monster’s room and then re-enters the room, he must make a another horror test. He must make a horror test every time he enters the monster’s room (and every time the monster enters his). In this way, a monster that is chasing an investigator will trigger a horror test each time the monster enters the investigator’s room.
  
  If, at the start of an investigator’s turn, a monster is in the same room as him, the investigator does not need to make a horror test. If a monster leaves an investigators room and then re-enters it on the same turn, he does not need to make a horror test against it. 
  
  An investigator only makes a maximum of one horror test per monster per turn.
  
  Horror tests always interrupt the current step of the player’s turn