Hi! It depends because each question is weighed differently...but if this helps I've gotten 40-50 q's wrong and have scored in the 800's. Ironically, my hesi score was higher (>850) for the one I got 50+ wrong than the one with 40+ wrong. If math is a weakness, do some practice questions with that because those are weighed heavily...also doing NCLEX practice questions and reading the rationales for both your right and wrong answers is also very helpful. Others also swear by Evolve and their case studies...good luck!
Also keep in mind that there's 5-10 practice questions that aren't counted. You won't know which questions those are so they'll come up as right or wrong but they won't be included in your score. I got 10 wrong on one of my HESI and ended up with an equivalent score of 96%.
In my experience you can only study so much for the HESI a lot of the questions are critical thinking so they aren't always straightforward. You need to know your stuff and how to apply the knowledge into clinical practice.