Do I need 'the?'

Yes, that's right. (with 'the')

sodawater You don't appreciate (the) bread you buy at stores until you make it yourself.

The is optional. You don't make store-bought bread yourself, so "bread" is more of a general idea than a specific loaf.

Thank you for your suggestion.

Some stores sell exactly the breads they make themselves, for instance hand-made breads.
In this sentence I would surely use "the".

I've learned that if you the noun in question are too many to count, it's rare to use 'the.' Therefore, I thought 'bread you can buy at stores' are too many and genera to count, so I thought it's better to just say 'bread' instead of 'the bread.' Is my interpretation wrong?

sodawaterIs my interpretation wrong?

I don't agree with it. Not sure where you're getting this information from.

As AlpheccaStars pointed out, the definite article in the original example is optional. Think of it this way:

You don't appreciate the bread you buy at stores until you make it yourself.
= You don't appreciate

Thank you for your wonderful examples.

Franky12In this sentence I would surely use "the".

Sometimes the use of articles is best answered by a native speaker because there are many tricky situations.
Are you a native speaker? Are you saying "the" is mandatory?

Thank you very much for your help, AlpheccaStars.