Way too many vertices when switching to edit mode i am trying to create a rock by sculpting an ico sphere, but even after small amounts of sculpting, when i switch to edit mode it creates around 100,000 vertices. does anyone know how to reduce the vertices without losing the shape? i dont mind starting again on the rock by the way. 
here is a screenshot (the rock is around the size of the starting cube)
Shrinkwrap modifier could help you:


Create new object with similar shape, big enough to cover your sculpted rock
Subdivide it as you would like the mesh of your rock to be detailed (or use Subdivision Surface modifier so you can tune it later)
Apply Shrinkwrap modifier with Target set to your high-poly sculpted rock.




You should get low-polygon version of your high-poly sculpted model.
There are some useful options you can use for tuning the Shrinkwrap modifier so you can look at it's overview in Blender Manual.

https://docs.blender.org/manual/en/dev/modeling/modifiers/deform/shrinkwrap.html