How to calculate the power of equipments connected to a voltage regulator?
The equipment you connect should be marked as to the VA. Add them all up and they should be less than the 500VA that device is capable of. There might also be problems if some equipment draws large surges, for example when starting.

There is no problem running many devices from the output- only the total consumption matters.

If you overload it it may overheat, the fuse may blow or it may fail early. If it's badly made it might emit dangerous fumes or present a fire hazard. 

The device appears to be a servomotor-driven variable auto-transformer (Variac) so it will take some time to adjust for mains variations (maybe a second or two). Whatever is connected must be able to withstand a surge or brown-out of that duration without damage.