One of my favorite exercises is the pullup. There are a lot of ways to vary them up. Here are just a few.
Change how you grip the bar. How your hands face affects how many pullups you can do.
Generally, overhand grips (palms facing away, properly known as a pronated grip) will take your biceps out of the movement. That focuses the emphasis on your back, which has to carry the rest of the load. Underhand grips (palms facing you, the supinated grip) bring the elbow flexors into the movement. This makes for a better bicep curl than the bicep curl itself is. Your back muscles still are fully involved, but your biceps can aid the movement. You can probably get an extra couple of reps with this grip. A third alternative is the neutral grip, where your palms face each other. This still involves the biceps, but not as much as a supinated grip, nor as little as pronated grip.
Finally, you can use a mixed grip. This means one hand in one grip, the other in another. Usually this means pronated and supinated, but it's possible (although somewhat awkward) to pull with one neutral and one pronated or supinated hand. Alternating hands helps counteract the rotation of a bar during a deadlift, and with a rotating pullup bar it can do the same. It can also just provide a form of variety.
Change the width of your grip. Try widening out the grip to make pullups more difficult, or narrowing them to make them easier. You can do as wide as a bit more than shoulder width without much problem supinated, and as wide as your elbow-to-elbow span pronated. You can narrow the grip as far as your hands touching or even overlapping. Generally a narrow supinated grip will be easier than a narrow pronated grip. neutral grip falls between these.
One warning - some people with shoulder problems have trouble with wide grips. Try them with care. If they don't bother your shoulders, you can keep at them. If you feel pain or discomfort doing wide grip pullups, try moving the grip in. It's a variation, not an essential movement. If your shoulders require you to stick with a shoulder-width grip or narrower, just stay there and do that!
Give those options a try next time you do some pullups, and add some variation in your workout.