Does 6-8 sentences means that i have to write 6 or 8 sentences
or
it means something else.

Well it basically means you:can write or type or whatever yoir doing either 6 sentences or you can do 8 sentences. you do you

6-8 sentences means you can write 6 throughout 8 sentences.

Explanation:

You can write a minimum of 6, a maximum of 8, or if you're feeling a bit spicy, you can write 7.

go to google if u want the answer