In English, "bread" is not countable, so it has no plural. If you want to count bread, you can loaves, or slices, or pieces, etc.

"you can <em>say</em> loaves...."

Sorry about the typo ;)

It is one of many "mass nouns" or "uncountable nouns." I think the idea is that they are just some kind of <em>stuff, </em>material, rather than being individual <em>things</em>. You turn them into <em>things</em> by using a phrase, such as "a loaf of" or "a piece of."

Here are some other examples:

That is a very big flock of sheep. There a hundred sheep in the flock: fifty ewes and fifty rams.

The water in the world's oceans has a total volume 1.338 billion cubic kilometers. I wonder how many drops of water that is?

As Dan says, there are indeed many non-countable nouns, as well as nouns that may be countable or not

-- How much pizza did you eat?

-- I had three slices (of pizza) / i had two whole pizzas = two pizza pies

Abstract nouns can be non-countable as well:

-- Some advice.

To count them, we can use piece "a piece of advice" or a countable synonym "a tip."

"Sheep" is always countable, it's just that the plural has the same spelling and pronunciation as the singular....