Generally, there are two, but the so-called "ball joint" is just a connection form of movable joint. As long as there is relative motion and it is necessary to connect, the ball joint connection mode is one of the possible design schemes, so there is no definite number for this. Of course, the most common one is the steering rod, followed by the suspension thrust rod.

Automobile ball joint: the outer ball joint refers to the hand pull rod ball joint, and the inner ball joint refers to the steering machine pull rod ball joint. The outer ball and the inner ball are not connected together. They have to work together. The ball head of the steering machine is connected to the sheep horn, and the ball head of the hand pull rod is connected to the parallel rod.