With the online version, one account, one return. Period. If you do a 2nd return on the same account, then the first return will be over written and lost forever, with no possibility of recovery. But you can create up to 5 online accounts per e-mail address.
When using multiple accounts, it's important to use the same exact account each year for the person that a tax return was prepared on last year. Otherwise, you will be importing the WRONG information from the prior year's return.

Next to your screen name above it says "Deluxe for Windows" which is a desktop product (CD/download).  If that's what you are using, then you can prepare multiple returns with the software and get up to 5 Federal efiles.

But if you are asking about an Online TurboTax account to prepare a return online, then see Carl's info above.

Yeah, I was typing that additional info up when you posted @epo39b as I realized I left it out before clicking the "comment" button. 🙂

For Online, Online is only good for 1 return.  But you can use the same email address for 5 accounts.   How to start another return in the Online version      
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Each ONLINE account can only prepare one return for one person.  Additional returns require separate accounts, and will be charged separate fees, unless you qualify for a simple free tax return.

The DESKTOP program that is installed on your own computer from a CD or downloadable installer can prepare an unlimited number of returns, with the first 5 eligible for e-filing.  (The license agreement says you can use it for personal use -- i.e. family -- but you can't charge people to prepare thier taxes using this version.) (Or any version, actually.)

It's  asking for a Service Code - this is the second return under the same email address (with different user ID)

Do I have to pay again? Or is the Service Code the ticket??

Yes the online version is only good for 1 return per account.  So yes you have to pay for each return unless you use the Free Edition.  Thats why if you need to do more than one return you should use the Desktop CD/Download program.

 

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-onli...