Almost 80% of stars seen in the universe are Binary stars.What makes them so abundant in the universe? Why isn't there other numbers but exactly two that is abundant?
Short answer - conservation of angular momentum. Basically, if something big is spinning and contracts, it starts to spin faster and faster. Think of a ballerina doing a pirouette and pulling their arms in to accelerate the spin! So now imagine a giant diffuse cloud of gas collapses under its own gravity. It doesn't have to be spinning very much at all to end up spinning very fast when it collapses to star size. But thats actually a problem! If you do the calculation you find that the star spins so quickly its surface reaches escape velocity before it finishes collapsing. There are two ways around this: either you ditch the excess angular momentum in a massive planet like Jupiter or you split the star into two stars spinning around each other.
Harsh jain's answer is correct, but it only answers one part of the question, which is why some stellar systems have more than one star, not why so few systems have more than two stars.
If you get more than two stars in the system, it becomes unstable. That's the famous three-body problem that the science fiction book has its name (and part of its plot) from.
Two stars together form a stable orbit around each other. Mathematically, that means that if you know the positions, velocities and masses of the stars at a given point, you can solve the equations of motion for any given future time and predict their future positions.
For a system of three or more bodies, however, this is not possible. The equations of motion now depend sensitively on the initial conditions. In a two body system, a small inaccuracy in the initial conditions will translate to a small inaccuracy in the predicted future position. You can trace the inaccuracy as it grows over time, and correct for it. With three bodies, the inaccuracy in initial conditions will result in an inaccuracy in the future predictions that grows exponentially with time, meaning that after a certain time span, there is no connection between your predictions and the actual positions. This is basically the definition of chaos.
One important consequence of this is that the there are no stable orbits in such a system. Sooner or later, two of the stars will either collide, or the bodies will interact in such a way that the combined gravity of two of them will accelerate the third to escape velocity and kick it out of the system, after which the two remaining ones will settle down in a stable binary orbit. The same is true for systems with a number large than three.