What is the best way or recommendation way to set up workflow approver for someone that will be associate to a lot of workflows? How do you hander when approver leave the company?
This should depend very much on the process you are working on I would have thought. If you have a single or group of approvers that are always the same, or rarely change, then I would suggest creating a SharePoint group, add your approvers to this group and then assign your task to this SharePoint group. This means when your approver leaves you don't need to republish the workflow, but simply change the membership of your group. This means you don't need someone who knows Nintex to make the change, just someone who knows how to add and remove users from SharePoint groups.
If, as I've had in the past, you have a whole list of approvers and the approver required depends on a bunch of conditions, then I tend to add these approvers to a SharePoint list and allow the workflow to do the work of selecting the approver based on the criteria (metadata properties) of the item.
Hope this helps, if so, please mark it as useful or an answer.