What is the safest, most reliable number of motors to use in each stage of a rocket?

Using one motor in each stage of a rocket is deemed the safest and most reliable approach. This is primarily because a single motor reduces the complexity of the launch system and its associated risks. With one motor, there’s a straightforward launch profile focused on the independent performance of that motor. Each motor is designed to provide a specific amount of thrust, and using one ensures that if it operates within its intended parameters, the rocket will behave predictably.

Additionally, having just one motor per stage minimizes the potential for failure modes associated with multiple motors. In scenarios where more than one motor is used, such as in tandem or cluster configurations, the potential for one motor to fail can lead to complex interactions and unpredictable behaviors. If one motor in a multi-motor configuration experiences an issue, it can destabilize the entire flight, leading to a much higher risk of mission failure.

This principle firmly anchors itself in the foundations of rocketry where reliability, especially during the initial stages of flight, is crucial for safety. Therefore, employing a single motor design simplifies the rocket's performance parameters, making it the safest and most reliable choice for any stage of a rocket.