how many dnd monsters do I have to read up on to survive what's the most common enemy number and terrain

[Naofumi]

 

[Naofumi]

 

[Magonia]

 

[Magonia]

common enemy number is 1v1​

IE 4 players vs 4 NPCs

terrain is according to theme​

Usually forest or dungeon type terrain
Usually the DM gives you enough clues to know what to expect as far as terrain or of you will fight an unusual amount of special monster types