###HUMAN: find the number of patients on additive type drug prescription who have diagnoses icd9 code 78791., ###ASSISTANT: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN diagnoses ON demographic.hadm_id = diagnoses.hadm_id INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id WHERE diagnoses.icd9_code = "78791" AND prescriptions.drug_type = "ADDITIVE"