The width of a trench that a ride-on trencher can dig varies depending on the specific model and its capabilities. Generally, ride-on trenchers can dig trenches with widths ranging from 4 inches (10 centimeters) up to 24 inches (61 centimeters) or more. Some models may offer interchangeable digging chains or attachments, allowing for adjustments in trench width according to project requirements. It's important to consult the manufacturer's specifications and guidelines for the particular ride-on trencher you're using to ensure it meets your project's needs.