As many as you’d like. You can opt for multi-line phone systems ranging from two to six lines and beyond. However, this choice depends on your business’s size.  A two-line system may work for you if you’re a solopreneur. This helps you prioritize your calls and appear more professional in your customers’ eyes.  On the other hand, a four-line system should do the job if you run a small business with multiple teams, like a sales and customer service department. This helps your teams communicate effectively, while each can prioritize and handle external calls without any issues.  You can also set up a dedicated fax line while you’re at it.  Lastly, six lines or more are recommended for large companies. For example, they can set separate lines for each project manager and each team member to ensure maximum effectiveness.