I believe you're asking about the motion resolution then? As in how fine the motion is, which does play a big part with the result of curved motion.

Motion vectors are plotted with a resolution of milliseconds. Given that an arc is just a set of straight-line moves that keep changing directions to make the actual arc, when you do a curve, the straight lines are as small as a millisecond (axis directions are changing every millisecond) making the arc very fine as long as you're moving at a reasonable velocity. By reasonable velocity, I mean setting the velocity accordingly to the material being cut, or if you're just freely moving.

In your example, Mach will not necessarily generate 20,000 vectors for the trajectory because the number of vectors generated will be dependent on your configured motor speeds and accel (slower speeds generate more vectors because it takes more time to get to the final position). To give you an accurate example, if your machine takes exactly 5 seconds to move from the initial to target position, then you'll get 5000 vectors plotted for the trajectory. That being said, you'll get your 20,000 vectors IF the motion takes 20 seconds to finish.

As a general rule, the more vectors are plotted for your trajectory, the finer the trajectory, meaning a slower move with an arc will generate a finer curve. But even if you were to command the motors to go full speed on an arc move, the resolution is still very fine.

Counts per unit is a somewhat irrelevant factor in the trajectory ONLY IF they are accurately configured for the motor.