I could answer this question in two words: “not very.”
I don’t mean to say that spelling is completely unimportant for TOEFL essays, but it’s only one part, and it’s not one of the most important aspects. Most of the time, spelling does not cause any problems. It’s rare that I see an essay from a student in which the spelling caused more problems than other aspects. The most important areas to pay attention to are usually other areas: structure, grammar, vocabulary, details, etc.
Partly, this is true because everybody makes some spelling mistakes, and they’re sometimes just a sign of being rushed. After all, you have to write your TOEFL essays in very short time periods (20 and 30 minutes). Even the best writers will misspell words when writing quickly.
So you can make spelling errors and get a perfect score. You can make several, in fact.
But all this depends on two aspects:
1. How frequent are the spelling errors?
If you have five spelling errors in your TOEFL essays, it’s not important. If you have 50, that may cause a real problem.
There is no special number or percentage for how many mistakes you can make. It’s a spectrum—the more mistakes you make, the more possibility there is for communication problems. So although spelling errors aren’t a major problem, you want to minimize them. If the reader has trouble understanding your essay because there are three misspelled words in every sentence, you will lose points. Don’t be completely careless! Keep in mind the reader—the TOEFL is a test of communication.
2. How big are the spelling mistakes?
Did you accidentally switch an “i” and an “e” so the text says “conveneint” instead of “convenient”? That’s not a big deal. But if you wrote “counveinnt” instead, that is a problem. If you write a word completely incorrectly, the reader might not be able to figure out what you meant. And if that happens, you will lose points for not communicating.
Usually, when I see essays in which the spelling does cause a real problem, there are at least three words that are so misspelled I can’t understand them at all.
Realistically, don’t worry about it
If you are worried about your spelling during the exam, you will hesitate and waste time. Never spend more than a few moments trying to decide the spelling of a word. If you are unsure, keep moving. Write what you think it is, and move on. But at the same time, don’t let yourself make mistakes that you know are mistakes. Both be careful enough that you minimize mistakes and don’t spend valuable time deciding when you’re unsure.
Instead, take that time to think about the more important parts of your essay: how specific are the details? Do the sentences and paragraphs transition well? Do you include all the important information from the lecture? Are your sentences complete and well-formed? Those are the areas to best spend your energy on.