We'll assume that you only want to talk about whole numbers.
The smallest 9-digit number is 100,000,000. The largest one is 999,999,999.
They represent (all of the counting numbers up to 999,999,999) minus (the first 99,999,999).
That leaves 900,000,000 numbers.
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10,000 of them, from 0000 to 9999. However, most people would not count numbers with leading 0s, so that there would be 9000 4-digit numbers.
9,000
24 of them.
100 to 900
823543