Currently, you can't put more than 16 text boxes into a simulation. I'm not sure why, though it was probably to prevent excessive use of them.
If you use too many signs, you will get downvoted, and possibly reported based on the new rules. There's no need for a limit on the number that can be on the screen.
Often, with complicated saves, or saves that rely on text (such as homepages), more than 16 are needed. I think that we should be able to put in as many as we want, as long it makes sense to put in that many.