Retopology is a skill and a topic of its own. It all depends on what you want to do with the model if you retopolgize or not.
For a game, as Omar said, Yes most definitely.
For a single image render to show your friends, No
There is no “amount of faces” that is the right number, it depends on the model. If it has lots of detail or even if it will be animated like a character for example.
This raise other topics too,
Texturing – are you going to need to UV unwrap to use textures
Baking – Often small details will be baked to a normal map from the original sculpted mesh and used on the retopologized mesh.
You will learn a lot about retopology by learning poly modeling and also animation. With animation you need edge loops correctly placed to help with the bending and deformation of the model, specifically character models.
I hope this answers your question, and you will learn more about retopology as you become a more advanced modeler.
.
