All of them, but it does depend on what operators are allowed.
How many numbers are there between 0000 and 9999?
You may notice that card from a certain institution all have the same first four number. how many different credit card numbers can this institution create.
The four digits can be used to produce infinitely many different numbers if repetition is permitted. Without repetition, there are 24 possible numbers. A lot more can be produced if the numbers are combined using binary oprations, fore example, 19 * 8/4 = 19*2 = 38.
If the digits are all different then 18. Otherwise, 192.
3*3*2*1 = 18 of them.
any 4 from 7 is 35, in any order is 35 x 24 ie 840
Only four numbers:20233032
With base one million, you can create one nonillion different numbers; using the traditional decimal (base ten) system, you can form 100 thousand different numbers.