In counties of the second class there may be appointed not in excess of 9 county investigators, who shall be paid annual salaries of not less than $6,500.00.
Not more than 6 county investigators in counties of the second class with populations of 400,000 inhabitants or less, and not more than 21 county investigators in counties of the second class with populations in excess of 400,000 inhabitants, in addition to those provided for in this section may be appointed by the county prosecutor where there appears to be a reasonable necessity therefor, if approved by resolution of the board of chosen freeholders of the county.