A triangle has three sides in total. If it has more than three sides, it is not a triangle. One of the three sides is called the 'base' ... usually the side that the triangle is resting on.
The base has three sides.
Five One base and four sides
An octagon is a plane figure. It has eight sides, but no real base.
A cone only has a base, no sides
4 sides
2 sides, the base and the cone.
5 sides
One side will be a square. It is the base. The lateral sides are triangles.
5... base and one triangle on each side
4 triangular sides and 1 square base
The base of a trapezium is one of the parallel sides. Conventionally it is the longer of the two parallel sides but that need not be the case.