Can I enter multiple years of taxes at one time..Or do I have to enter and send them separately

No, each tax year must be a separate return.  You can only use TurboTax online for the current year, but you can purchase the prior years as a downloaded program.  

Each tax year is completed and filed separately.  The TurboTax online editions are Only for tax year 2024

If you need to complete prior year tax returns using TurboTax you will need to purchase and download each tax year required to your personal computer with an OS of either Windows or Mac.

Go to this website for the prior year tax returns - https://turbotax.intuit.com/personal-taxes/past-years-products/