Depending on several criteria, generating numbers via the form will likely be absolutely painful. Below are just a few of the "let's start slow" questions you should try to answer before proceeding down the path of Unique Number Generation Using The Form

1. Does the number need to be unique?
   
   
   
   
   
   • If so, can you *actually* just get away with using the ID?
     
     
   
   
   
   
   
   



2. Do users absolutely have to be able to assign their own number sometimes?
   
   
   
   
   
   • If they do could they input those User Assigned numbers into a different column so there is a distinction between the two?
   
   
   
   
   
   

The reason I ask this is because SharePoint generates an incremental and more importantly, unique, number for every Item (The ID), and it's very simple to just utilize that instead of trying to come up with your own method of generating unique numbers in an organization where there is any potential of two users possibly trying to create a new item at the same time.

If a workflow were to run upon Item Creation, then it could easily grab the ID and turn it into a meaningful unique and to-your-organization's-standards number, without having to do any form work at all.

Let me know what you're answers are, and we can perhaps proceed from there. 

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yes the number should be unique, and its exactly like you said "If a workflow were to run upon Item Creation, then it could easily grab the ID and turn it into a meaningful unique and to-your-organization's-standards number, without having to do any form work at all." 

but the question will be how to turn it into organization standard? 

>> but the question will be how to turn it into organization standard? 

To answer that is difficult because everyone is different. What do the unique numbers look like now? Is there a way we could approximate their "style" but have it be based off of the items ID rather than whatever it was before? 

Without more information it will be difficult to answer.