I would say a 600L or larger tank should suffice for a year or two if they are small to medium in size.
In my 6ft i have 7 yellows and two purples for three years now and in my 5000L tank i have 15 yellows, 11 purples, 1 black, 2 scopas, 1 hyrbrid scopa, large naso and two salfin tangs and...