1188 of them.
Two
You don't - you need additional information. Many different rectangles can have the same diagonal. If you know the diagonal and one side (which must be LESS than the diagonal), you can use Pythagoras' Theorem to calculate the other side.
9 diagonals
A pentagon has 5 diagonals.
Parallelogram
A diagonal cannot be a side of a rectangle, and a side cannot be a diagonal.
There are four diagonal lines in a hexagon.
A heptagon has 4 diagonal lines and 7 sides and it has 2 congruent diagonal lines and 2 congruent triangles
1188 of them.
Not many. Depending on how you write, the letters L, O, Q, and X can have diagonal lines of symmetry.
4
zero
it has 4 :)
14
14.
It has one.