Audio designers should consider the breadth of their "palette" of sounds, similar to how artists consider the range of colors and textures they will use. The number of different audio clips does not, by itself, define the variety of sounds. How greatly the sounds vary in pitch and in texture matters as well.
Humans rely mostly on their sense of sight, which is why players are more likely to notice bland or redundant art than bland or redundant audio in a game. Many players have complained about the drab and repetitive visuals of Fallout 3. I put off buying S.T.A.L.K.E.R. largely for the same reason. Players are less likely to pinpoint their displeasure if sound effects run together, but it can also be a significant issue.
Like with art, audio should have a general style. But shake it up. The sounds of weapons shouldn't just sound cool, but also be distinct from each other. The sound of footsteps is more immersive when the player can occasionally hear a creaky board, a puddle, a bit of hard clay, etc. Shouts and exclamations, like in Call of Duty 4 multiplayer, are more interesting when players hear a variety of voice of different depth and aggressiveness.
Shake it up. Dynamics are important in everything, including sound.