How to submit an extra tax return to the IRS? the software only allows five returns.

Follow the instructions here to contact support. If you use TurboTax Desktop, you can file five federal returns with up to three state returns each.

The 5 efile limit is from the IRS. It's to prevent fraud and people filing fake returns. To do more you have to sign up as a ERO - Electronic Returns Originator and be an authorized E-file provider. And you can't buy another Desktop program to get more efiles.

See if one of them can use the Online Free Edition or use the IRS direct file or one of the other free files listed at the IRS.

IRS Direct File
https://directfile.irs.gov

IRS Free File partners
https://www.irs.gov/filing/irs-free-file-do-your-taxes-for-free

You can print the additional tax return and file it by mail. The limit of 5 tax returns applies to e-filing, not to filing by mail.

Intuit will not be able to help. It's the IRS that imposes the 5 return limit. After that any additional returns will have to be mailed.