How are you going to Milford Sound? If you are driving you need a night in Te Anua. If you are taking the long one day bus trip from Queenstown you can skip that. Either way is fine, when I did it I drove but the bus trip would have been OK also. The other place to stay is Wanaka. There is plenty there to justify one or two nights. So I would suggest 2 nights Wanaka and either 4 nights Queenstown or 3 nights Queenstown and one night Te Anua. How you string that together depends on flight arrival/departure times. But 6 nights in Queenstown would be ok too, if you don’t mind long drives.