The answer isn’t necessarily black and white as there are quite a few variables in this. However, generally a startup should look at hiring at least a part-time person in HR when the company has around 15 employees. They don’t need to be full time, but at least get someone in to address the HR needs. It’s always easier to get policies and procedures implemented when the company is at a smaller scale and then scale those policies as the company grows than it is to try to implement a policy when the company has hundreds of employees.