If your numbers look like this 12345-01 then you can use a string function called LEFT to get the first five digits and then use that to do COUNTD(). In your case, LEFT([Your Number], 5) .
Online Help (String Functions):
Returns the left-most number of characters in the string.
LEFT("Matador", 4) = "Mata"
Just to add to your helpful answer...
It may work but as the left function uses a string as an input, Jonathan may need to wrap the 'number' field first in STR() to convert it to string, if the field is a measure/ number.
LEFT(STR([Your Number]), 5)
And extending that...
I doubt you need this, but just in case - if your aggregating the 'number'' field, as below
You will need to add the aggregation to the calculation, like this -
LEFT(STR(SUM([Your Number])), 5)
Note - I only used 3 characters in the screenshot, its not the same formula!!
Hope that's clear!