How to add thousand decimal in number of task email ?

You should write your email on a workflow.

Take a look at  

Maybe on of these posts will help:

How to format number with a comma

Number format in Nintex forms

Forms - Display numeric fields with thousands separators

Eric Rhodes‌

Thanks for your suggestions. I have formated correctly both Nintex Form number fields and Sharepoint columns. They show the correct number with thousand separator as below image.

Fernando Hunth‌

But when I insert the variable was stored number into the email body, it was not have thousand separator, I have used "Build String" to build custom HTML body

I think the possible way is "regular expression" but I have tried many methods but it was not working.

In your workflow, try adding a "Set Variable" action and configure it like the screenshot below.  It takes a number list column, formats it as currency, and saves it into a text workflow variable that can be used in the email. Make sure to click the three dots (...) beside the Field selection to select the return type.

Here's a good post to use if you need to remove the decimals: How to email currency with thousand separator but not decimals

Eric Rhodes‌

Thanks for your tip. I have done successful. Based on your tip, I have figure out the simple way.

- Build String > Advanced Lookup > Current List Item > Select the column contain the number > Select return type > Formatted Currency (as image)

Thanks all very much.