You need to deactivate the CD Key from one of the computers currently using the CD Key before you can activate another computer or purchase additional FlexODBC license seats.

See: How do I deactivate FlexODBC? How do I transfer my license to my new computer? 

If you have been moving your QODBC license from a work computer to a home computer and then back to your work computer you will eat up your seats.

For example: Computer A to Computer B to Computer C will use one seat.

But Computer A to Computer B to Computer A will use two seat counts because Computer A has been licensed twice.

Note: Your deactivation may take 24 to 48 hours to process depending on your region and license pack.