I do not do batch extensions. You get no confirmation number.
I open each return, add 4868, fill in appropriately (usually all 0) then efile the extension like I do a return. This way you get an acknowledgement number when it is accepted.
If there are problems down the line, that you must have proof that the extension was filed, nothing but the electronic confirmation number will do. Not even the green card from mailing them certified.
CYA Do them one at a time in each extended return.
A word to the wise is sufficient.