There are a number of HR Metrics and Workforce Effectiveness surveys out there that you provide a reference for an employees-to-HR employee ratio. At minimum, I'd advise that you be very careful in making apples-to-apples comparisons from this type of metric...for example, it may be appropriate for a manufacturing organization/department to have a higher employees-to-HR employee ratiothan a sales organization/department. Also note that these numbers may vary widely based on the HR operating model (if you use outsourcing heavily, for example, this will inflate the ratio).
Ultimately, does the organization care how many HR employees there are? I think, no...rather, the organization is interested in the value HR is generating for the organization for the investment cost. To me, this means you have to be able to demonstrate how HR is doing in both value creation and in cost management.
Cost: I find that internal comparisons are the most effectuve. One metric I like is "total HR spend as a % of revenue." Compare last year to this year. What does the trend look like over time? Does the data suggest that HR being responsible with its spending and is increasing its productivity into the organization?
Value: I find that a combination of program effectiveness measures serves pretty well as a starting point. For example, what is your organization's effectiveness in terms of attracting, developing, and retatining talent? Are employees and managers satisfied with their jobs, their development opportunities, and their HR generalists? How is compensation and benefits spend relating to revenue generation over time? To what extent is compensation cited as a reason for turnover? If you know how well you're performing as an HR organization, you can then use that information to compare where your performance is versus where you want to be, and what what needs to be done to bridge that gap.