We have been at WDW 5 times this year & we eat at TS every day. With checking in on the app, it tells you if you can add a person or not. It will say “you can add up to 1 person” or “up to 0 people“. It very much depends on the restaurants capacity at the time of your reservation. If they are full, they won’t let you add anyone. If they have space, they offer the option.
It is a very bad idea to book for less people thinking they’ll let you add “just one” more. This question comes up at least weekly, if not several times a week. A lot of people think they’re the only ones who thought to do this, but it’s a lot of people trying to add an extra guest. So many that they added that info on adding a guest on the check in page. Especially now with limited TS availability, you may get turned down. Have a backup plan just in case.