Just a quick one....

Have a Sales Id which is unique , but can I reset this...

Also how many records can we add in a table

many thanks

First, number of rows is not really a limit, the size of the MDB is, depending on the version of access I think you are limited to 2 GB or 4GB for the newer (not 100% sure on that).
You can reset a key, issues are how other tables have that key stored to tie back to the table. Is it unique, you must generate unique then. The best method is to create an external (seperate) table that is a cross reference to your index and would contain the old value, and the new value. Research everywhere the value is stored and write queries to update those values. It is most likely better to drop the index/key and recreate it once the values have been changed.