In the experimental setup you've described, where a car is moving along a table with one end of a string attached to a fixed number of metal discs and the probes are measuring acceleration and velocity, the quantity that is likely to stay constant during the trials is the mass of the system (the car and the discs).
Assuming there are no external forces acting on the car (like friction) changing the total mass, and that the number of metal discs attached to the string remains fixed throughout the trials, the mass of the car-plus-discs combination will be constant.
Additionally, if the trials are conducted under the same conditions (same initial velocity, same setup, etc.), other factors such as the tension in the string could also be considered constant if the same setup applies throughout the experiment.
However, note that the acceleration and velocity of the car will vary depending on the forces acting on it (like gravity, tension from the discs, and any applied forces).