If you can fill many of the basic human resources management needs internally, and partner with experts where you have a gap, then you can likely reach a hundred or so employees before you need to think about hiring an HR Manager. My first HR Manager role was with a company of over 250 that went more than 100 years without an HR Manager! Various managers and even VP’s were responsible for overseeing aspects of HR, and they used their legal counsel to create an employee handbook. They hired me because they were facing some EEOC and OSHA issues and finally decided they needed someone on the inside to manage HR.
An HR consultant friend of mine worked 20 hours/week as an HR Director for a company that wasn’t ready for a full-time HR person but wanted someone with high-level HR skills to help them build a solid HR program. They have now grown to a size where they were ready for a full-time person, but my friend was able to put processes in place and provide HR advising for many years.
For more information on HR Management check out my book A Small Business Guide to Peak Performance Through People.