I've been managing projects for 15+ years for big organizations, small companies and my own personal projects. Based on the scope of your projects, it sounds like a single project manager could simultaneously manage multiple projects for you.

An important consideration is to find someone who's style of project management fits well with your goals, team, your company culture and your clients. For example, if your projects are smaller in scope, you don't need to put the same type of project management system in place that a multi-million dollar global system upgrade would require. You want to make sure that the project management layer helps get things done more effectively and efficiently, rather than just add another layer of administration and overhead.

One way to choose the right person for you, whether that person is intended to be an employee or consultant, would be to find 2-3 candidates, then ask each to develop a sample plan (or part of a sample plan if it's a big project), for the same project (or part of a project), then ask the project manager to walk you through the plan and how they would manage it. Doing this gives you an idea of each project manager's style and how it fits or doesn't fit with your goals, team, company culture and clients. I'd think expecting no more than an hour of work along these lines from potential candidate would be a reasonable request.

Hope that's helpful and happy to discuss further on a call!