How many unique triangles can be drawn with side lengths 4 cm, 6cm, and 8cm?

I think you can make two of them.

As you walk around the triangle in the clockwise direction,
you could encounter the sides in the order of 

                     4cm  ==>  6cm  ==>  8cm,

or in the order of 

                     4cm  ==>  8cm  ==>  6cm. 

Those two possibilities are mirror images of each other,
so they're not congruent, and they're different triangles.