In formal writing, we never begin a sentence with a numeral. Instead, we spell the word out. For example:
Ninety-two percent of the students graduated, and 78% of the students attended college the next year.
However, we often try to avoid this by rephrasing. For example:
An impressive 92% of the students graduated, and 78% of the student attended college the next year.
In your example, however, “Ninety-six percent real meat” isn’t even a complete sentence. If you are writing something formal, you could say, “This product contains 96% real meat.” If you are simply putting a phrase on a label, etc., advertisements typically use all caps:
96% REAL MEAT