What number of 7.5 hydrocodone pills is equivalent to 250 ml liquid hydrocodone 7.5?

Depends on brand: preparations differ in their concentrations, and the amount should be on the label. If it was prescribed for you, the pharmacist can explain exactly how to use it. For most liquid preparations, the amount (hydrocodone 7.5 mg and Acetaminophen 325 mg) would be in one teaspoon or 5ml, so 250 ml would be 50 doses. If it's NOT prescribed, you shouldn't be taking it, and should seek drug treatment.