What is the plural of the number 7?
I would avoid trying to put a plural on an actual number. It looks goofy and is not proper to write 7s. Use the word instead: "Seven."
Then it's easier to make plural. Most English nouns can be made into a plural simply by adding an "s." That makes your word "sevens." Used in a sentence: "Their address has multiple sevens in it."
It would be great if the English language always worked by adding an "s" to form a plural, but there are a lot of irregular words to contend with in addition to the words that you add "es" to instead of "s." For example: "child" to "children"; "woman" to "women."
Then there are the super weird ones where the plural and the singular are the same thing: 1 deer; 8 deer.
Seven is not irregular. It doesn't end in y, ch, x, or s, so it doesn't need an "es" ending. Use "sevens."