What is solitary flower?
Solitary flower means one flower at a specific position of a plant. It usually accompanies with another word either terminal or axillary. 

Soliary terminal means a flower present at the apex of the main stem or its branches. e.g.  Trillium grandiflorum.


 Image taken from http://ohioplants.org/inflorescences/ 

Solitary axillary means a flower arising from the axil of a leaf. e.g. China 
rose.



Image taken from https://commons.wikimedia.org/wiki/File:China_rose.JPG


 This is Sida rhombifolia. This picture is clearer.

No. It doesn't mean that a plant has a single flower.It speaks all about the position of the flower. Think of roses , they are solitary terminals and are in numbers in each plant, always growing at the apices of branches.

Source: My two years of learning Botany as an additional Undergraduate subject.