In the CD/Download version of TurboTax, you can prepare as many federal returns as you want. You get 5 free Federal e-files, there is a charge to e-file a State return. TurboTax personal software is not for professional tax prepares, it is against the licensing agreement. You can install the software on any computers you own, up to 5.
The Online version you can prepare one return per account. If you use the same account (UserID & password) to prepare additional returns, the second return would overwrite the first, & so on. TurboTax Online or the mobile app to prepare multiple returns, but each return requires its own unique login and must be paid for separately (if the return doesn't qualify for Free Edition).
