One stone has two grits, one on each side. Rarely will a 220 grit be needed if you have a power grinder, and the 8000 grit is only needed when you want a mirror edge, such as when using an infill plane. We like to get that mirror edge, but we also realize that is not for everyone. If that describes you, then this stone is all you will ever need. The combinations stones are available in 3 combinations.
Most people use the wheels that came on the grinder.
These wheels are the worst possible choice. They are almost sure to burn the edge of a properly tempered tool. The trick is, that knowing the grit of a grinding wheel is not enough. The hardness of the wheel is also critical. The wheels that come on a grinder are very hard, designed to be long wearing while removing stock quickly.