I am assuming that you want to automatically assign a unique account number to each customer. And then I suppose that you aslo will want this to be represented on the order, in the admin, on the emails etc.

This will take a good bit of added code and an addition of a record to the customer details table to house this number.

If you still want to tackle this you can look first to how the order numbers are auto generated/incremented and take from this the required code and validation that will be required to be inserted into the create account areas.

Now that this has been created, you will have to follow this through with code additions that check and include this account number in the same manner that the customer name is included.