I have a database that I want to implement an encryption per x amount of users.

Say I have 1000 Users in the database and I wanted every 1000 encrypted differently then I would generate a new key and use it for users 1001 - 2000 that registered. this would then be repeated for every 1000. What I can't figure out is the best way to do this and I am trying to do this with hopefully 0 changes to the database as to give as little away should my database get compromised.