One for each application.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
One I-797 Receipt Notice is generated by USCIS for each application. If your petition was a single one (i.e I-539) where both minors were cited in the single extension petition then you will only receive one official receipt notice applicable to both minors. Follow your attorney's guidance.
Response to this question does NOT establish an attorney-client relationship with the person requesting such information.