In English, “bread” is uncountable. We say:
Do you want some bread?
If you want to count bread, you have to use one of the following counters:
- a piece of bread can be any size
- a slice of bread is a flat piece that can be used for toast or for one side of a sandwich
- a loaf of bread is the “whole bread” - the bread as it looks when it comes out of the oven
We use “loaf” when we talk about buying bread:
I bought a loaf of bread, some milk, and some apples.