The easy answer for the sake of the rollers is a minimum of six (6), that's (18,000lb pipe divided by 3,000 lb capacity rollers). However, you should also consider the pressure area of the roller against the pipe and potential for pipe deformation at those locations. Further, you should run a beam calc to check the unsupported length against bending.
Unfortunately the OP does not inform of the pipe thickness or length. Once you have those, you can check the shear force against the area and the moment against the section modulus.
I used to count sand. Now I don't count at all.