So do not use your real account/return to test in. You can use the same email for 5 accounts. You don't have to pay for an account unless you want to print it or file from it.
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...
The Desktop program can do unlimited returns and has other features and advantages over online. You can copy your return and it has a What-If worksheet you can use.
