What should be the cumulative dose of isotretinoin for moderate acne?is it 120mg/kg like the severe acne or less?can i relapse if i take less than 120

Yes: According to the American Academy of Dermatology statement, high dose isotretinoin ( 120-150 cumulative) is as effective but senior in preventive relapse than regular or low dose.

See Dermatologist: Unless you are super certified most doctors can no longer prescribe this medicine. 120mg /kg seems like a massive dose to take. Please see the doctor that is prescribing this medicine and you're more likely going to get better clarification of this medicine.