Count nouns are things which can be counted. Such things are thought of in terms of separate or individual items. One way you can identify a count noun is that it has a singular and plural form. Another way is that it can be used with numbers.
Mass nouns are always in the singular form.
However, many concrete nouns are also mass nouns because they are thought of as single substances rather than individual items. These are often gases, liquids, and materials which do not have any definite boundaries, and solids which consist of small pieces. Most nouns are not always count or always mass. They usually fall into more than one category, depending on the way they are used in a particular context. That building contains 16,000 bricks.