Does the studio have copyright after paying? He asked to take a photo as a demonstration. What should I do?

The photo studio paid you the right to use your photos and portraits. You sell it to him, sign an agreement, and he can use your photo for publicity. If you don't want him to see it, you can not sell it. If you collect money after signing the agreement, it depends on their mood. If they don't use it, they won't show it. If they do, they can show it directly without asking your advice!