See IRS Handbook for Authorized IRS e-file Providers of Individual Income Tax Returns page 34 #4 and page 36 #2 for limit of 5 efile
https://www.irs.gov/pub/irs-pdf/p1345.pdf#page36
Additional Requirements for Participants in Online Filing
In addition to the above, participants in Online Filing must adhere to the following:
1. Ensure that no more than five electronic returns are filed from one software package or one e-mail address.
2. Supply a taxpayer with a correct Submission ID.
