What objective are you working towards?
The data in a a table is not stored in any particular order therefore to number records without imposing a sort on the table will lead to sporadic results.
If you are seeking to create a unique reference use an autonumber field, it won't always be incremental but it will give a bookmark to indicate record input, again you would need to consider how to sort the data.
An append query would probably be your best bet by importing all the records(via a sorted query) into a new table with an autonumber field.
Ian