Creating a scripted field is a two-step process. We first need to create an instance of the custom field in Jira, and then add the script to it:

1. Log into Jira with a user that has a Jira administrator's permission.
2. Navigate to Administration > Issues > Custom fields.
3. Click on the Add Custom Field button and select Advanced from the dialog box.

4. Scroll down and select Scripted Field from the list; click on Next, as shown in the following screenshot:

5. Name our new custom field Total Comments and add it to the appropriate screens.
6. Navigate to Administration > Manage apps > Script Fields.
7. Select the Edit option for the Total Comments field.
8. Enter the following Groovy script in the script text box:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.comments.CommentManager 

def commentManager = ComponentAccessor.getCommentManager()
def numberOfComments = commentManager.getComments(issue).size() 
 
return numberOfComments ? numberOfComments as Double : null

9. Select Number Field for Template and click on Update, as shown in the following screenshot: