There's only one global mid-ocean ridge system, but it's interconnected and stretches for over 65,000 km (40,000 miles) around the world. It's not a single continuous line, but rather a series of segments that make up the longest mountain range on Earth.

While there's just one global system, it's often referred to as having multiple "ridges" based on geographic location, such as the Mid-Atlantic Ridge, East Pacific Rise, and the Southwest Indian Ridge.

So, the answer depends on how you define "mid-ocean ridge". There's one global system, but several individual segments within it.