Assuming all 45 digits are different (which they will not be if they are ordinary integers), then 456, if repetitions are allowed. That is 8303765625.
If repetitions are not allowed then the number falls to 45*44*43*42*41*40 = 5864443200
180 of them.
500
There are 34 = 81 numbers.
pizza
27
There are 7,290 different 4-digit numbers that can be formed from the digits 1-9 without repetition.
There are 3024 of them.
180 of them.
There are 60480 numbers.
840
24
500
27 three digit numbers from the digits 3, 5, 7 including repetitions.
There are 5*4*3 = 60 such numbers.
1 set
There are 5! = 120 such numbers.
The number 1234567890 can create about 39 of 40 4 digit numbers.