I just applied to every project i thought looked interesting, then if further down the line i saw that it was not as interesting as I thought, I withdrew my application. This way, I always had projects on the go, and was getting a feel for how phd projects work, it also kept my options open.
There is no limit to how many projects you apply to, just be prepared to fill out a lot of forms. One thing I did was have a ready email to fire off to everyone, this made it quicker to contact universities.
Some universitys are very slow at processing applications, so by applying to loads, you will get a good mix of slow and fast responses, and so hopefully a load of interviews you can attend, this way you will be able to compare the projects and see what ones you like best.