This is a general purpose Auto Numbering plug-in for use with workflows for any CRM entity.
The workflow plug-in is a dll file named as: ASK.AutoNumber.dll which needs to be registered using registration tool PluginRegistration.EXE
Both files, ASK.AutoNumber.dll and PluginRegistration.EXE should be downloaded and saved in this directory:

C:\Program Files\Microsoft Dynamics CRM\Server\bin\assembly

Download PluginRegistration.EXE AutoNumber.dll  zip file  

Step 1: registering ASK.AutoNumber.dll
Run PluginRegistration.EXE as administrator
Set up connection to Microsoft CRM (two steps process)

Step 1:

Step 2:

Allow few minutes for the plug-in registration tool to discover all CRM Organizations on the CRM server and the already registered plug-ins for each organization.
A plug-in is registered for an organization.
Select the organization where you wish to register the plug-in.
By selecting an organization, the internal window displays all registered plug-ins & custom workflow activities for the organization.

Click Register on toolbar & select “Register New Assembly”

Browse & upload the file ASK.AutoNumber.dll

Click> “Register Selected Plug-ins”

A message will confirm when the plug-in is registered

Step 2: Creating Auto-Numbering workflow
For the purpose of demonstrating the auto-numbering workflow, we use the Request For Change (RFC) entity that can be downloaded here.
We would like the workflow to create an increment number for the attribute with displayed name RFC ID # (attribute name: new_name)
Before you start deselect the read-only option of the attribute (customization>forms)
Create new workflow for the entity Request for Change with workflow name: AutoNumber
Set the start when to “record is created” and make it available to run “On demand”

The workflow has 3 steps:

1. Define how the auto-number is generated
Click>Add Step and select the plug-in “Auto Number”
Click “Set Properties”
Enter:

Prefix - up to 6 digits
Number – the integer part which will be served as the first auto number
Attribute – the attribute name (not the displayed name)

2. Update attribute in Request for Change entity

Click>Add Step and select “Update Record”
Select the record as “Request for Change”
Click>”Set Properties”

Don’t forget to set the attribute to read only to eliminate modifications to the stored values (Customization>forms) 
 
If you wish a failed workflow to send you an email with error message
Click>Add Step and select “Check Condition”
Add the following condition:

Click>Add Step and select “Send Email”
Add these fields to the body of the email and send it to the record owner

Save

Publish workflow & test it