Technically you don't need one for sculpting; A mouse functions in sculpt mode. However if you want to sculpt regularly with Blender a tablet is highly recommended. It's far more artistically intuitive both for sculpting and texture painting if you get into that skillset as well.
Here's an article we wrote about this subject.
When you start learning sculpting, a mouse is all you need to learn the basic techniques and the effects of the different settings.
A tablet offers you additionally the ability of dynamically changing certain brush setting like "Radius" or "Strength" depending on how intensively you press your pen against your tablet which is the called "Pressure Sensitivity".
Screenshot from Blender 2.90: Marked by me with a yellow frame: The blue "Pen" button signalizes that "Pressure Sensitivity" is acitivated for dynamically changing the brush's strength whereas the grey "Pen" button indicates that the brush's radius doesn't change. Maximum pen pressure means that the value in field for a certain parameter is applied so that you won't see any effect from your pen stroke if you set all these pen pressure controled values to zero.