Ok, upon further investigation I have found that the normal complement is two ball bearings per side of each track, one at the front, one at the back. ie there are 8 in total, 4 per side.
Each ball bearing has a limited run of about 10 cm, held in place by some crimps on the outer and inner runners.
The problem I am seeing is that if the balls get to the front or rear of its travel on both ends of the runner, then the seat is only supported on each side by two point contacts about 10 cm apart. This is why my seat always rocked back of forward a lot. Best case would be a much wider base of about 30 cm.
The bearings are about 6mm, but I'd need to extract one to measure, which may not be easy. If I added more per side, the same problem may exist as all the balls could group at the front or back of their run.
Other seat runners use a sliding carrier to hold the ball bearing where they should be.
I guess with the seat in place there is enough weight on the bearings to stop them sliding back and forward. The answer to the problem is to add sticky grease to prevent noise, then make sure the bearings are all centered in their travel before installing the seat.