This is a keycap with the number 9 written on it. You can see the keycap on the keyboard
⌨️ or telephone dial pad
☎️. It can represent the number 9. It can also be used as a substitute for some words with a similar pronunciation to 9. Other keycap emojis:
#️⃣*️⃣0️⃣1️⃣2️⃣3️⃣4️⃣5️⃣6️⃣7️⃣8️⃣🔟