From experience, 4 sinks. If, an when you get the bug to venture out and do festivals, what is required in your area changes to another in another area. My HD warned me and purchased mine with 4 sinks. Plus easier to sell anywhere.
Sinks are easy to build, plumbing in can be a pain routing hoses where there might not be room. Not much room under some carts.
Need fresh water tank and gray water tank. Size of tanks matters in some areas. Gray water is always larger than fresh.