1 at each vertex
An exterior angle of a polygon is an angle formed by a side of the polygon and the extension of an adjacent side. In the case of a triangle, each exterior angle is formed by one of the triangle's sides and the extension of an adjacent side.
When an exterior angle is formed at a vertex of a polygon, the measure of the exterior angle is equal to the sum of the measures of the two interior angles adjacent to it. In the case of a triangle, the sum of the measures of the two interior angles adjacent to the exterior angle is always 180 degrees (which is the sum of the measures of all three interior angles of a triangle).
Since each exterior angle of a triangle is formed by two interior angles, and the sum of the measures of those interior angles is always 180 degrees, there can only be one exterior angle at each vertex of a triangle. Therefore, a triangle has one exterior angle at each vertex.