The only employer I remember seeing statistics for had one HR person for every fifty-six employees (in that country - it was an international bank and I was not in the HO country), and there were about 3,000 employees.
That, I recall, was considered relatively HR-heavy, and that around one HR person to every eighty to one hundred employees was considered typical.
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This is my opinion; it is not legal advice, nor the view of my employer, and it may change tomorrow.