A number type is used for numeric values. You would give variable n a numeric value the same way you would a string type variable, like:
<cx:let name="n" value="123">
... $n contains the numeric value 123 ...
</cx:let>
With fractions, use a decimal point, like in the following:
<cx:let name="n" value="15.99">
... $n contains the value 15.99 ...
</cx:let>