How many digits are needed to create a number in the hundred millions?

100,000,000 = 9 digits, unless you're going to get sassy and say that 'create' can also mean calculate the number through f.ex. multiplication.

Then you'd quickly be down to at least 3 digits, as 10^9 = 100,000,000 = 9 digits.

Whether or not 2 digits would be possible I am not entirely sure, then I guess you'd have to enter the world of symbols and letters (thus loosening the definition of 'digit' as well).