Take a pair of zeros and place them together.
00
Note there are zero digits between the pair of zeros.
Now place a pair of twos around them.
2002
Note there are two digits between the pair of twos, and still zero digits between the pair of zeros.
It's straightforward to construct strings with this property using [only] the remaining even digits.
420024
64200246
8642002468
Let's include the odd digits, [Edit] and eliminate the zeros.
For how many of these groups of digits, using the digits in pairs, can you construct strings with this property? 1
1,2
1,2,3 - clarify: for this case, arrange 1 1 2 2 3 3 to have one digit between the 1's, two digits between the 2's and three digits between the 3's
1,2,3,4
1,2,3,4,5
1,2,3,4,5,6
1,2,3,4,5,6,7
1,2,3,4,5,6,7,8
1,2,3,4,5,6,7,8,9
Have fun.