Question: how many different 7 digit combinations can be made?
Answer: In this case you have all ten digits [0,1,2,3,4,5,6,7,8,9] at your disposal. Assuming repetition is allowed, you can have 7-digit numbers from 1,000,000 to 9,999,999 which is a total of 9,000,000 7-digit numbers. These are all the possible 7-digit numbers. In general, there are 9 × 10^(n-1) possible n-digit numbers.