A number is a count or measurement.
They are really an idea in our minds. We write or talk about numbers using numerals such as "5" or "five". We could also hold up 5 fingers, or tap the table 5 times. These are all different ways of referring to the same number.
There are also different types of numbers, such as whole numbers (1,2,3) decimals (1.48, 50.5), fractions (1/2, 3/8), and more.