Many players develop a small but effective set of serves, and find success with this. However, often they are limiting themselves with a lack of variety.
While it is important to develop a small set of highly effective serves that you can use over and over, it's also important to have enough variety in your serves that you can most likely find one that the opponent has trouble with.
For example, many players develop very nice forehand pendulum serves, the most popular serve in high-level table tennis. But since so many players do this serve, many opponents are good at receiving them, while having trouble with other serves. I've seen many players who have trouble with a specific serve, such as a backhand serve short to the forehand. (This has the opposite sidespin as a forehand pendulum serve, though it can be mimicked with a reverse forehand pendulum serve.) And then when I coach against this player, I find that the player I'm coaching can't do this serve! Often they can do the regular forehand pendulum serve short to the forehand, but it's the other type of sidespin that the player has trouble with.
Another example would be fast and deep serves. Many players have difficulty with certain types of these, such as fast and dead to the middle (elbow). And yet many players can't do this simple serve, and so are giving away many points, games, and possibly a winnable match. Fast sidespins to the wide backhand give many players trouble, and yet few bother to learn these serves. And then there are the all-out forehand loopers who loop every deep serve to the backhand - and woe be the server who can't cross up this player with a fast serve down the line to the forehand!
The list goes on and on, and yet the principle is simple. Learn to serve short and long (including fast) to all parts of the table with all varieties of topspin, sidespin, and backspin (and don't forget corkscrewspin and no-spin! See article on spin if you're not sure about these two), and you'll have it all covered. While you don't need to be an expert on every serve, you should be at least proficient at most serves so you can pull out the needed ones when needed.