Analysis: 9.9 million written as 9.900000 is a seven-digit number. Five 0.99 million should be added after 99 = 9900000. Therefore, five zeros added after 99 is 9.9 million; so the answer is: 5. Comment: This question tests the reading and writing of integers. The key to answering this question is to find out what number 9.9 million is.