Values of Number custom fields or number constants may be formatted in email templates in various ways.
The key to number formatting is the $!number variable which is an instance of the NumberTool Jira API class.
NumberTool provides buit-in styling for formatting numbers but also provides access to Java NumberFormat API which provides ultimate formatting capabilities.
Formatting with built-in styles
Below is an example of formatting the value of a number field called "My Number Field" in various styles.
Format with Java Number format
Below is an example of using Java's NumberFormat to render the number value in Japan's currency: