There’s nothing gramatically or structurally wrong with the phrase you’re planning on having tattooed: “Fall seven times, stand up eight.” Plus, it’s a positive message promoting perseverance and determination.
It’s worth noting, however, that “fall seven times, stand up eight” is actually a Japanese proverb roughly translated into English words. It may carry more meaning in its original form and context.
Good luck! Let us know how it goes. 🙂