Question

How many corners are in a rectangle and how many flat faces

Answer 1

I think you mean rectangular prism
Anyways a rectangular prism has 8 vertices (what you call corners) and also 6 faces.<span />

Answer 2

Rectangle is a polygon (A close plane figure which has 3 or more sides)
Rectangle is a parallelogram (Opp Sides are parallel and congruent)
Rectangle has 4 right (90 degrees) angles, 4 sides and vertices (Corners).
Just saying a square is a type of rectangle and rhombus (4 Congruent sides)

Search about Quadrilaterals. Glad I could help!