Hey User999
you can use the same sort of approach you use to dynamically generate a GUID (e.g. ${=java.util.UUID.randomUUID()}) as to generate a unique number based on the current timestamp or current date, etc., there's also a Math.random() method you can use too.
I'm on the wrong laptop so I'll check later (my ReadyAPI is on a different laptop) - but my question --> what's the max field length of the unique numbers you want to generate?
Also - I think (going from memory here - I've never actually used it) the DataGenerator (DataSource step option) will also generate unique numbers for you.
Cheers,
Rich