If a parallelogram has a right angle then it is a rectangle?

No because it could be a right angled trapezoid.

A rectangle must have four right angles.

Yes, a parallelogram with a right angle is always a rectangle. In fact, in my geometry book, some 55 years ago such was the definition of rectangle.

A parallelogram with one right angle will always have four right angles.

The right angled trapezoids that are not rectangles are not parallelograms.