You need to strike the right balance between having enough staff to keep the business growing, and keeping the company lean.
Planning is important in determining how many staff you need. Establish goals for your business, break those down into checkpoints and then determine what staff would need to do each day to reach those goals.
If it becomes clear the workload is simply too much for one staff member, then consider how a second would affect the workflow. You don’t want staff sitting around doing nothing, especially when a business is first starting.
Once you start reaching a point where it would be cheaper to add to your staff than continue operating at the same level, bring on another employee. This will allow you to keep bringing in clients and servicing work at a pace that will allow you to grow comfortably.