**Explanation:**

In geometry, an altitude of a triangle is a line segment from a vertex of the triangle to the opposite side, which is perpendicular to that side.

- **Definition of an Altitude:** An altitude of a triangle is a line segment that connects a vertex of the triangle to the opposite side or its extension, and is perpendicular to that side.

- **Properties of an Altitude:**
- The three altitudes of a triangle are concurrent, meaning they all intersect at a single point called the orthocenter.
- The orthocenter is not necessarily inside the triangle. It can be outside the triangle in some cases.
- The length of an altitude can be found using the formula: *Altitude = (2 * Area of the triangle) / (Base of the triangle)*.

- **Number of Altitudes:**
- A triangle can have three altitudes. Each altitude is drawn from one of the three vertices to the opposite side or its extension, forming a perpendicular line segment.
- The intersection point of the three altitudes is called the orthocenter. Since the three altitudes are concurrent, they all intersect at the orthocenter.
- Therefore, the correct answer is option C: 3.

In conclusion, a triangle can have three altitudes, and the altitudes are concurrent at a single point called the orthocenter.