Usually, they are added in ppms (few hundreds as maximun)

200ppm, 500ppm, 700ppm - max.

Ashphaltene precipitants vary in behaiviour and activity. Their inhibition also should differ. so the thresshold concentration of the inhibitors depends on the precipitant used.

You will find your best answer at the lab. Asphaltene's molecules and deposition model vary in each oil independently on the Asphaltene's content. Start with  a low dosage, say 100 ppm

please can you tell me what are the disadvantages of using asphaltene inhibitors or where gone inhibitors after injection (losses)???

Very varys in types of crude, and generally may be about 50 to 500 ppm by concentration of the chemical

Agreed with

@ Mohammad ebrahim Rahimbeiky