Ah... I should have started back at the top of this thread to see this has already been answered in an earlier post:
The IRS limits anyone who is not a licensed tax preparer to five electronically filed returns.Anything beyond five must be printed, signed and mailed to the IRS. Additionally, each individual return must be in a physically separate envelope, each meeting it's own postage requirements separately.
and
Ensure that no more than five tax returns are filed electronically by one software package or from one e-mail address;
And the rationale is to make it harder for tax scammers to make multiple fraudulent returns.
The original question (at the top of this thread) didn't quite clarify the need to exceed the 5 return limit but it sounded like it was someone needed to file returns for a large family and wanted to efile them all.
This topic is discussed in other threads too ... e.g.
https://ttlc.intuit.com/community/taxes/discussion/can-i-file-more-than-one-return-with-my-turbotax-...
