Here is the most basic example of how to add two numbers in Jinja.

sum = {{ 1 + 1 }}

It is much more common to store the result in a variable.

{% set sum = 1 + 1 %}
sum = {{ sum }}

Which should return the following.

2

Increment

Here is an example of how to increment a value.

{% set count = 1 %}
original count = {{ count }}

{% set count = count + 1 }}
new count = {{ count }}

Which should produce the following.

original count = 0
new count = 1

An increment is often used as part of a while loop.

{% set count = namespace(value=0) %}
{% for item in list %}
  {{ item }}
  {% set count.value = count.value + 1 %}
{% endfor %}