Only the manufacturer can tell you how much additional weight is acceptable. It is not so much the case that the weight would strain the motor's capacity to spin at the proper speed--significant extra torque would only be needed at startup even with a massive weight; it it the extra load on the bearing that would be of concern. That extra weight could shorten the life of bearing or increase the amount of vibration (noise) generated by the bearing.
If you are concerned about the weight, you should look into clamps. They don't weight as much and can do a better job of pressing the record tightly to the platter surface. In particular, reflex clamps do a good job. These clamps grip the spindle, and as you turn the knob on top, they apply increasing amount of downward pressure on the record label area. To work as intended, a reflex clamp also needs a washer or o-ring around the spindle to actually raise the record up a little bit so that when downward force is applied beyond the washer or o-ring, the record is slightly bent; this bend causes a much larger area of the record being pushed into contact with the platter.
Ideally, clamping and added weight is applied in conjunction with the design of the platter/mat. The shape of the surface matters, and not all systems are designed to work best when the record is held in more forceful contact with the platter/mat. Like everything in audio, one has to take trial and error risks; nothing is a universally positive tweak.