How many sheep you can fit per acre depends totally on the type and quality of the land and grazing. The absolute max quoted for lush pastureland is 6 whereas for some land it's not sheep per acre but acres per sheep.
I think you have plenty of homework to do before you take delivery of your animals. Try to vist other small scale breeders to see their systems, visit agricultural shows in the summer to see the breeds and talk to their owners.
On half an acre you would have to start off very small and stay very small, until you can acquire more land. You would also be best to keep one of the smallest breeds, such as Soay, Ouessant or maybe Shetland, and just a couple of ewes if you want to go for it.