For example, One such 6-digit number is 502294, 
another is 209955, and still another is 999999.
We can choose the same digit more than once.

The only restriction is with the first digit.
The first digit cannot be 0.  So we choose the
most restricted things first.

We can choose the first digit as any of these 
4 digits {2,4,5,9).  
For each of those ways,
we can choose the second digit as any of these 
5 digits {0,2,4,5,9). 
For each of those ways,
we can choose the third digit as any of these 
5 digits {0,2,4,5,9). 
For each of those ways,
we can choose the fourth digit as any of these 
5 digits {0,2,4,5,9). 
For each of those ways,
we can choose the fifth digit as any of these 
5 digits {0,2,4,5,9). 
For each of those ways,
we can choose the sixth digit as any of these 
5 digits {0,2,4,5,9). 

Answer 4×5×5×5×5×5 = 12500 six digit numbers.

Edwin