The "PhoneNumber" parameter can contain a comma delimited list of recipients. There is no limit to the number of recipients supported imposed by NowSMS ... however, any library that you use for performing the HTTP request may have a URL size limit.
Alternatively, you can create distribution lists via the NowSMS web interface ... and then you just specify the distribution list name in the "PhoneNumber" parameter.
If you want to manage/edit these distribution lists external to the NowSMS web interface, please see the following link for information: