This is not a cut and dry type of answer. It really depends on how many openings you have, what level those opening are (non-exempt, exempt, manager, director, etc.), how "rare" or "common" the skill set you're looking for is, if it's a relocation position, etc. If you are an internal recruiter and working for a large corporation then your work load is probably 20 to 30 openings, at varying stages, at any given time. Therefore 8 to 12 hires per month would be about right. If you work for a midsize company, on average you probably have 10 to 15 openings and 6 to 8 hire per month is about what you'd expect. Smaller firms, (300-500) employees I'd expect your monthly job openings to average around 5-10 positions, therefore 0-5 hires per month. It really depends on so many factors and circumstances that I might think an average of 5 hires per month is great, and someone down the street will think that sucks! I've recruited for every scenario and size company I've described above and those are about what I used to average.
Hope this helps!