What is the maximum number of secondary shares you could sell and still retain more than 50%

Question:

What is the maximum number of secondary shares you could sell and still retain more than 50% ownership of the firm? How much would the firm raise in that case?

The firm you founded currently has 12 million shares, of which you own 7 million. You are considering an IPO where you would sell 2 million shares for $20 each.