It's not about pressure, it's about gpm (gallons per minute). Now pressure will likely effect how many gpm a head is going to output, and you should be able to find tech specs on the heads you plan to use that will likely tell you the gpm based on pressure at the head. So what you've got to do is determine the gpm available to your system, and from there its simple arithmetic to determine the number of head you can use. Of course don't forget to allow for a little extra (assuming the water feeding your irrigation is the same feeding your house).
I also agree with the previous posts that irrigationtutorials.com is what you need to read. There is alot, and on first read will be a little confusing. But since you are asking a pretty basic question, it shows you either need to pay for someone knowledgeable to design a system for you, or you'll have to pay by way of time to give your self an education. If you don't you're likely to put together a system that doesn't work and it will cost even more to fix after the fact. So one way or another... you're going to pay.
The good news is that if you attempt to read the tutorials and still have questions, there are several knowledgeable people in this forum that will be happy to help you educate yourself.
