I have heard that a person raising a commercial herd, can run twice as many cows as a person running a purebred herd. I tend to agree with this, when you consider all the additional things that need to be done when operating a purebred herd properly. Part of the answer to your question will lie in how much time you are willing to devote to your herd. Like has been said earlier, it will depend on several other factors as well, such as how far your pastures are from your home base, how much feed you have to put up, how long you can graze the herd, and many other things.
For many years my dad and I ran 350 cows ( of which 250 were purebred Shorthorn and 100 were some other purebred breed and/or commercial) and also had 800-1000 head in our feedlot. Both of us enjoyed working with cattle, but when I think back to those days, I cannot imagine ever doing it again. Most days we were outside long before daybreak and we never came in until long after dark. The two main things we did were work and sleep. My dad is now almost 89, and the hardest thing he has to deal with is not working all the time. He still comes out to the farm mainly from spring to fall and tries to do something around the yard. He really misses working with the cattle, and he will oftentimes drive out and just spend some time with the cattle in the pastures. It has been his life and it has been hard for him to do anything else. Personally, I am hoping to eventually find out if there actually is life after cows. ( I will probably still own some when I die, but hopefully someone else will be looking after them)