InfoPath Dev

 How can i acheive,  any example will be appreciated.

I need to distingguish the request number when the user click the link in his email, as per link i will approve or reject the item.

In the SharePoint Designer when you are editing the workflow, you can get the link of the current item. It's a bit tricky but it is definitely available to you.

In the Define E-mail Message dialog, click Add or Change Lookup. Select Workflow Context as your data source and Current Item URL. Click OK.