Take a look at this post
http://compacttractorreview.com/cs/forums/post/1463.aspx
It's an old pic put should give you the info requested.
My windshield is 2 pieces so I used 8 o-clamps. If yours is a single piece, you may be able to get away with 6 but 4 will probably make it more stable.
When you get the clamps, there will probably be insets inside the part that goes around the pipe/ROPS that makes them usable on 1.5" pipe. Just pull those out.
The clamp has a little T-handle that is used to open the clamp so it can be wrapped around the pipe. There will also be a 3/8" (or so) stud that sticks out which is where you mount your "stuff". This is the stud that you will bolt your windshield to.
I put the clamps on the ROPS with the studs facing forward. I then marked where the plexiglass needs to be drilled (and, of course drilled it).
When mounting the windshield to the studs, I put in some rubber washers. The kind that are metal on one side and rubber on the other. I put them on both sides of the stud so the rubber part is facing the windshield material. Don't know if it was really needed but this allows a little buffer to: 1 - avoid scratching and 2 - a small compression zone for when you put the wingnut on. I believe I got these in those specialty drawers in the hardware section of Lowes or HD.
BTW - You may want to get a can of ROPS grey or just regular black Rustoleum. The pic in the link above is pretty old and all the hardware, wingnuts, washers... have all been painted. Here's a bit newer pic but it's still missing some accessories.