a porblem about sculptting

when i use sculpt mode then i should add multiresolution modifier . after do this i get a very high poly object its about one million poly how can i get low poly object (except smooth) . third picture show my problem .

You’ll need to retopoligize the model. Something that simple shouldn’t really require sculpting in the first place, however. It could easily be modeled poly-by-poly with extrusions and then subsurfed and you’d probably save yourself quite a bit of time.