Lines of Symmetry of Plane Shapes
Line of Symmetry
Here my dog "Flame" has her face made perfectly symmetrical with a bit of photo magic. The white line down the center is the Line of Symmetry 
Folding Test
You can find if a shape has a Line of Symmetry by folding it.
When the folded part sits perfectly on top (all edges matching), then the fold line is a Line of Symmetry.
Here I have folded a rectangle one way, and it didn't work.
So this is not a Line of Symmetry
But when I try it this way, it does work (the folded part sits perfectly on top, all edges matching):
So this is a Line of Symmetry
Triangles
A Triangle can have 3, or 1 or no lines of symmetry:
Equilateral Triangle (all sides equal, all angles equal) 
Isosceles Triangle (two sides equal, two angles equal) 
Scalene Triangle (no sides equal, no angles equal) 

3 Lines of Symmetry  1 Line of Symmetry  No Lines of Symmetry 
Quadrilaterals
Different types of Quadrilaterals (a 4sided plane shape):
Square (all sides equal, all angles 90°) 
Rectangle (opposite sides equal, all angles 90°) 
Irregular Quadrilateral 

4 Lines of Symmetry  2 Lines of Symmetry  No Lines of Symmetry 
Kite 
Rhombus (all sides equal length) 

1 Line of Symmetry  2 Lines of Symmetry 
Regular Polygons
A regular polygon has all sides equal, and all angles equal:
An Equilateral Triangle (3 sides) has 3 Lines of Symmetry 

A Square (4 sides) has 4 Lines of Symmetry 

A Regular Pentagon (5 sides) has 5 Lines of Symmetry 

A Regular Hexagon (6 sides) has 6 Lines of Symmetry 

A Regular Heptagon (7 sides) has 7 Lines of Symmetry 

A Regular Octagon (8 sides) has 8 Lines of Symmetry 
And the pattern continues:
 A regular polygon of 9 sides has 9 Lines of Symmetry
 A regular polygon of 10 sides has 10 Lines of Symmetry
 ...
 A regular polygon of "n" sides has "n" Lines of Symmetry
CircleA line (drawn at any angle) that goes through its center is a Line of Symmetry. So a Circle has infinite Lines of Symmetry. 