The choice of a whetstone for sharpening a katana depends on personal preference, experience, and the condition of the blade. However, a common recommendation for sharpening a katana is to use a combination of water stones with different grit levels.

Water stones are popular for sharpening Japanese swords like the katana due to their ability to cut efficiently and create a refined edge. Here are some common water stones that are suitable for katana sharpening:

1. Coarse Grit Stone (200-400 grit): This stone is used for initial shaping and removing nicks, chips, or damaged areas on the blade. It helps in establishing the basic bevel and correcting any edge irregularities.
2. Medium Grit Stone (800-1000 grit): This stone is used for further shaping, refining the edge, and removing any coarse scratches left by the coarse grit stone. It helps in transitioning to a more defined edge.
3. Fine Grit Stone (3000-6000 grit): This stone is used for honing and polishing the edge. It refines the bevel, removes smaller scratches, and creates a sharper and smoother edge.

It's important to note that the specific grit levels can vary depending on the condition of the blade and personal preference. Some individuals may use additional intermediate grit stones for further refinement.

When choosing whetstones, it's advisable to select high-quality stones from reputable manufacturers. The stones should be flat, have consistent grit distribution, and provide good feedback during sharpening.

Additionally, it's recommended to use a stone holder or base to secure the whetstone during sharpening and maintain stability. Proper technique, patience, and practice are also essential for achieving a sharp and polished edge on a katana.