How many separate returns can I file using TurboTax Premier?

The TurboTax online editions are only for completing one tax return per account and User ID for any of the online editions.

The TurboTax desktop CD/Desktop editions can be used to create, complete and file as many tax returns as needed without a further charge after purchasing the software.

You can e-file up to five (5) federal returns using the desktop (CD/Download) version and virtually an unlimited number of returns to be printed and mailed.

The online versions are limited to one federal return per purchase.

Only 1 if you are using the Online web version.  Online is only good for 1 return per account.  But you can use the same email address for 5 accounts.   How to start another return in the Online version

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

The Desktop CD/Download program can do unlimited returns and efile 5 Federal for free.  Buy it here

https://turbotax.intuit.com/personal-taxes/cd-download/ 

If you started in the Online version and want to switch to the Desktop program we can tell you how.