The maximum payload of a star tracker is how much weight it can reliably support. This would be the combined maximum weight of your camera, lens and any camera accessories in use like a tripod head.
However, you'll also want to assess the load-bearing ability of your tripod, to check if it can cope with the weight of all the gear plus the star tracker. If in doubt, go for bigger capacity mounts because as a rule of thumb, it's best to have your rig's total weight about half the capacity of the mount.