If you sneeze 7 times in a row it means that you're gonna clear your nasal passages of all the germs that are trapped in the nose.
Sometimes it takes 7 and even 10 times of sneezing to get all the germs out of your nose because not all the germs come out after one sneeze.
There can sometimes be some more particles leftover in your nasal passages and by sneezing more times as in 7 or 10 times in a row you clear your nasal passages of all those bad particles.
I really hate it when I have to sneeze more than a few times in a row because it always seems to happen right in the middle of doing something else.
But if you do need to sneeze you should do so and not hold it back but cover your nose and use a tissue if possible to keep germs from flying all over the place.