How many do different countries have?

According to various sources:

As of July 2024, Russia has approximately 65 active submarines, making it the largest submarine fleet in the world.

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The United States has approximately 64 submarines.

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China has approximately 61 submarines.

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Regarding nuclear submarines (i.e., ships with nuclear propulsion):

The United States has approximately 66 nuclear submarines.

Russia has approximately 31 nuclear submarines.

China has approximately 12 nuclear submarines.

There are also sources estimating that 42 countries worldwide possess submarines.

Who is the "leader" and why?

Based on the data, it can be concluded that Russia leads in the number of submarines (approximately 65) with technical and strategic capabilities. The United States and China follow.
It is important to note that this number often includes not only the most modern submarines, but also those in reserve or undergoing repairs. They can have different engine types (diesel-electric and nuclear).

Why this is important: submarines offer a strategic advantage - stealth, the ability to attack from an unexpected direction, and the protection of sea lanes and infrastructure.

What should be considered in the analysis

The number of submarines does not directly indicate the technical level, combat readiness, or current capabilities of the fleet. For example, one country may have many obsolete submarines.

There are different types: diesel-electric and nuclear. Nuclear submarines have greater endurance and power, but are also more expensive.

The data is collected from open sources and may contain errors, especially for closed countries and fleets.

Conclusion

The submarine fleet remains a key element of naval power. Today, the largest navies by number of boats are Russia, the United States, and China. The United States, however, leads in the number of nuclear submarines.
If I had to choose one country as the "most powerful submarine force," I would put my bet on Russia based on the number of active submarines and the United States based on the quality and quantity of nuclear vessels