Is 18 toothpicks enough to form 5 quadrilaterals if no quadrilateral shares a toothpick with another quadrilateral?

Yes, if a generic square is made, and a toothpick along the vertical and horizontal line of symmetry, and then another generic square. 18 is enough to form it, and so is any double-digit number.

JK they cant share any. But maybe, if you use more than toothpicks