* "On page 10" is correct if you are referring to a specific page number in a book, document, or other written material.
* "In 10" could be correct if you are referring to a timeframe, such as "I will be back in 10 minutes."
Example:
* Correct: "You can find the answer on page 10."
* Correct: "I'll be back in 10 minutes."
Therefore, you need to consider the context to determine the correct phrase.