Difference between Digit and Number
Within the world of mathematics it is relatively common to confuse the terms number and digit. In fact, many times both concepts are interchanged and used indifferently. However, there are very important differences between the two that are worth knowing in order to avoid making this mistake. To begin with, a number is an abstract mathematical concept that represents a magnitude, a metric quantity, or an element within a number system. A number can be represented in the form of a digit or a figure, depending on its characteristics.
Figure 
Digit 


Definition  On the one hand, a figure is a graphic character or a symbol by which a number can be represented in writing. That is, the figure is the graphic representation of the number, not its value.  On the other hand, a digit is a number that can be expressed using a single digit numeral. It is each of the symbols used to express a number. 
examples  1, 2, 3, 4, 5, 6, 7, 8, 9, 10 […] 1000, 2000, 3000, etc., correspond to figures of the Arabic numeral system.  Within the decimal system there are 10 digits: 1, 2, 3, 4, 5, 6, 7, 8, 9, 0. The number 2874 is made up of the digits 287 and 4. 
other systems  A number of the Roman numeral system can be any of its characters: I, V, X, C, M.  In the binary system it is made up of two digits (0 and 1). The decimal system is made up of 10 digits. 