You can use math expressions to insert values into fields or set values of a number custom field anywhere where custom fields can be edited (create, edit & transition actions). Details of operators, functions and constants are in the math reference section.
In a number custom field you can use an expression to set a value:
1 + 4 sets the field to "5"
To use a math expression in another field, insert the value using:
{{#=}}1 + 4{{/}} // more verbose but more obvious {{#math}}1 + 4{{/}} // increase a number by 1 {{#increment}}4{{/}}
Referencing number fields
To perform calculations with number custom fields in Jira, you can simply reference the field as a smart value in your calculation:
{{#=}}3.2 * {{issue.My Number field}} + 43{{/}}
Or, if you wanted to add two custom fields you could use:
{{#=}}{{issue.Sale Amount}} * {{issue.Discount}}{{/}}