When it comes to choosing a yoga mat, thickness is one factor you may want to consider. A thicker mat can provide more cushioning, which may be beneficial if you have sensitive joints. If you have knee pain, a thicker mat may help to protect your knees when you practice certain poses.
There is no definitive answer when it comes to how thick your yoga mat should be if you have bad knees. Ultimately, it is a matter of personal preference and what feels most comfortable for you. If you are unsure, it is always a good idea to consult with your doctor or physical therapist to get their professional opinion.