How many tax applications can I file at one time?

When you use online TurboTax software you get one return per fee.

Each return needs its own account and user ID.   If you use the same account and user ID for a second return, the second one overwrites the first return and it is lost forever.  

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

If you have returns to prepare and file for multiple family members, etc. using desktop download software is more cost effective.  For the one-time purchase of the download you can prepare and e-file up to five federal returns and the corresponding state returns ($25 state e-file fee, or file state for free by mail).

The number of tax returns you can file at one time using TurboTax depends on whether you are using the online or desktop version:

• If you use TurboTax Online, you can file one federal return with up to five state returns per Intuit Account.
• If you use TurboTax Desktop, you can file five federal returns with up to three state returns each.
• No more than five Intuit accounts and five corresponding federal returns (for example, for a family of five) may be created, purchased, and filed using any one email address.
• Per IRS rules, you can only e-file up to five federal returns using the same email address or phone number with personal tax return preparation products.

For more detailed guidance, you can refer to the following sources:

• TurboTax Help Article on Filing Multiple Returns

The TurboTax online account is only for one federal tax return and up to 5 state tax returns per account and User ID.

See this TurboTax support FAQ for information - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/start-another-return-turbotax-online/L0B7Rjkyo_US_en_US?uid=m9fyyurd