Lots of Enemies Vs. Few Enemies

This is an interesting one, as while both types of games are fundamentally the same - you shoot stuff - in terms of game dynamics, they're very different. Games with lots of enemies tend to be very fast paced and frantic. Perfect examples of these would be the Serious Sam games, or a guilty pleasure of mine, Starship Troopers.

Masses of bugs in Starship Troopers
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Games with few enemies tend to be more suspense orientated, and the enemies while fewer in number, are much stronger, require more accuracy to kill or have a much better attack themselves than those in many-enemied games. Good examples of this would be FEAR, COD4 or Half Life 2.

A typical encounter with clone soldiers in FEAR
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"The Fear"

Fear is something that a lot of FPS games try to replicate, and although there are great games on either side of the camp, both achieve a similar result in very different ways. To explain this, I want to refer to a recent article on Bit-tech by James Silva in regards to a phrase of his, "the fear." This "fear," is an amalgamation of what you would traditionally call fear, along with tension, suspense and trepidation. The point he made, was that games with suspense and atmosphere are far scarier and therefore contained much more of "the fear" than those that threw gory enemies at you, and expected you to scared by just their visage time and time again. On this I totally agree with him; it's the reason that FEAR left me looking for new underwear, and Doom 3 just produced a light sweat for the first 10 minutes of each play session.

Alma uses her psychic powers from a distance
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However, that's not the only way to induce what he calls "the fear." Sometimes the scariest moments are when the enemy isn't hidden in shadows, but is in broad daylight and they just keep coming. You know in your heart of hearts that you are probably about to die, but you're going to hang on for as long as possible. It's still "the fear" but it's a different kind, as you aren't "scared" in the traditional sense. You aren't hiding behind a pillow, or looking at the screen between your fingers, but your heart rate is high, your hands are sweaty, neck hair is to attention and you seem to keep elongating expletives as you shout at the screen, "shiiii****."

Kleer skeletons close in; Serious Sam
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Level Design

Level design is something that's drastically different between the two types of game play. Those with lots of enemies, have, and require, large open spaces to give the player room to move, and to provide plenty of space of the hordes of enemies to attack from. However, there are often choke points allowing for concentrated fire from the player - or from enemies - adding some strategy to how you tackle certain areas.

Huge open environment for a Serious Sam boss fight
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Large open environment in Starship Troopers
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On the other hand, games with fewer enemies need to be much more enclosed to make it easier for the enemies to surround you. There are less avenues of escape, playing on everyone's natural instinct not to be enclosed, while also allowing many more opportunities for cover, IE. places to hide yourself.

Typical Call of Duty 4 building with pillars, tables, boards etc.
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