I expect the number needs to be unique, but several questions come to mind:
Does it need to be sequential?
How is it used?
Would 32 character UNID be too cumbersome?
What versions of Notes Client & Server are involved?
Is this a web application?
Can the number be assigned later by the server in the background?
Are there replicas of the database?
All these will have an effect on if & how this is accomplished.