can I complete multiple tax returns online with a one time payment or should I purchase a copy from the store

If you use TurboTax Desktop, you can e-file up to five federal tax returns. With TurboTax Online, you can file one federal  tax return and up to five state tax returns. 

Refer to the TurboTax Help article How many tax returns can I file with TurboTax?

Refer to the TurboTax Help articles: Can I file multiple returns with TurboTax? and Can I install TurboTax on more than one computer?