The number variable type is a container for whole numbers (i.e. those without a decimal place). The variable type can store both positive and negative values. The values that are accepted in the number field are 0 - 9 and the minus symbol, "-".
Using the expression field, you can set the initial value of your number variable. Note, the initial value is the "initial value" only, the variable can have alternative numbers assigned to it.
Please also be aware that if you assign a fractional number (a number with decimal place) to a variable of type number, the system will simply not store any value after the decimal place. The system will NOT round up the number.