restrict quantity of purchased items to a fixed number

I want to have a price for a single unit and then a price for 24 which is a case and that's it. if a unit is £1 and a case of £24 is £20 it is a £4 discount but if they order 23 it is still £23. if they order 25 it is not good for me as I would have 1 case and 1 unit so it should just be if they order 24 units or 48 units not 25 units.