Searching a in multimap for a range of keys

I am programming a College Library system. as books \ magazine database I using a map that the key is the Item ID number (1000000 - 99999999) and the value is a pointer to the item object.

each item related to one of 7 existing faculties on the collage and this is how the key of the item is set. the first digit is the faculty number ( 1 -7) and the rest of the number is 0 000000X (when x is a number relative to the items size)

my question is can I get how many keys are stating with the number (1 - 7) so I will know how many are in the specific faculty.

Thanks in advance. tzion.