If you choose to send Simple Report there is a limited amount of static parameters that can always be used:
Date Parameters: 
{{ ds }}  ==> today’s date                                 
{{ ts }}   ==> current timestamp                           
{{ yesterday_ds }}  ==> yesterday’s date                   
{{ tomorrow_ds }}   ==> tomorrow’s date                    
{{ ds | date_format('Today is %d, %b %Y') }}  ==> format using “Today is …“
{{ ds | date_add(7) }}   ==> adds 7 days to today’s date 
{{ ds | date_subtract(3) }}   ==>  today’s date - 3 days 
{{ ds | date_add(7) | date_format('Next week's date is %d, %b %Y') }} ==> sample mix
Mail Merge Reports allow you to load custom parameters using JSON, SQL, or a Tableau view. These Jinja-syntax conditionals will work on all your parameters.
IF statements:
{% if parameter > 0 %}
    {{ parameter }} is > zero 
{% else %}
    {{ parameter }} is <= 0
{% endif %}  
Loops:
{% for item in parameter %}
item is: {{ item }}
{% endfor %}
Sample Message:
Hi everyone,
Please find yesterdays ( {{ yesterday_ds }} ) report attached to this email.
{% for region in sales_region %}
    {% if region > 10000 %}
        {{ region }} sales were above 10000 yesterday! 
    {% endif %}
{% endfor %}
Best regards,
...
Templating Syntax
The general syntax to use parameters in the report template is {{ parameter_goes_here }} and is following Jinja templating syntax.
For example, if your JSON or SQL has a parameter named first_name 
 
you can apply it like this: {{ first_name }} 
Please make sure you don’t have spaces in your parameter names. A simple typo will result in an empty space in your report.
You can also use more advanced options to dynamically change the message.
JSON Parameters:
Assuming you have imported the same parameters as given in the 
Loading Parameters section. 
Your JSON parameters look as follows: 
Sample:
[
    {
        "client_name": "John Doe",
        "email_recipients": "john.doe@honda.io",
        "slack_recipients": "John Doe (U4FPP5UU8)",
        "filter_name": "Car_Brand",
        "filter_value": "Honda"
    },
    {
        "client_name": "Dave Doe",
        "email_recipients": "dave.doe@toyota.io",
        "slack_recipients": "general (C4FPP5UU8)",
        "filter_name": "Car_Brand",
        "filter_value": "Toyota"
    }
]
As you have TWO entries, this JSON will create TWO reports. One for John, one for Dave. 
Please Note: 
Your filters will only apply if you add your chosen value for “filtername” (here: “Car_Brand”) and the parameter {{ filter_value }} into your attachment.
See the Attachments section if you are unsure. Decorate your Message as follows:
    
For clarification; the message that John receives will look something like this:
Subject: 
Sales Report Honda, 2020-01-01 to 2020-01-08 
    -> {{ filter_value }}, {{ ds | date_subtract(7) }} to {{ ds }}
Message: 
Dear John Doe,  -> {{ client_name }} 
attached to this Email: Last week’s Sales Report for the Brand Honda.   -> {{ filter_value }} 
Best regards, 
…
