Will, you can start with one cow and a bull, if you want to. If you expect to make money, you'll need to build yourself a budget. What does the land cost you each month? Cows eat all year; will you have to supplement them with hay in the winter? Is there hay available in your area? What does it cost? Do you have water, corrals? You need to visit with your local Extension office and see if they can give you some help on a budget, information on how many head that land can support, etc. Where will you buy cows? Will you buy bred cows, pairs? Registered, commercial? Lots of variables here. Good luck...
I don't think there is a set number to have for being profitable. How many head you need to make a certain amount of money is entirely different.
50 cows will eat twice as much as 25 cows. 50 cows will need twice as much pasture as 25. And you will probably need 2 bulls instead of 1 with 50 hd compared to 25. Obviously a lot of costs are directly accumulated on a per head basis. But, if your investing money in corrals, feeders, barns, tractors, and other equipment, it would be better to spread that cost over more head. Also, if you figure your labor, you would be better of with more head. But if it is more of a hobby or part time deal, you really can't expect an hourly wage that would compare to working out.
So to answer your question, you will have to look at how much money you will be investing up front on equipment, etc. to determine if you can make money. If you already have most of what you need, you should be able to make a little money with just a few head.