No expert here but my understanding is this.
With a cell camera sending pics to email it actually stops receiving pics from other cameras while it’s connecting and sending via cell service. I’m not sure of the shutdown time. 5 or so minutes. Not a problem if you get very few pictures or have the time between pictures a cam takes set very high. If your cam is taking a pic every 1 to 5 seconds and a group of does is in front of it, it could be counter productive. The cell cam receives a pic, shuts down receiving and starts the process of sending an email.
If you have 30 pics sending one at a time the cell is shut down from receiving for 150 minutes 30x5 or 2.5 hours.
If your batch is set to 30 it only stops receiving for 5 minutes to email all of them. (Again my 5 minutes could off +/-)
Low batch numbers and high picture numbers could create a big log jam.
EDIT
Sorry just reread your post. Not sure if you have it set to send every 12 hours. Maybe it would send them all at that time rather than shutting down and starting the process over for each pic. But I have mine set to batch 31 and that’s how many pics I get with each email unless I’m getting the daily report.
I have batch set on 10, I get from 7 to 22 pictures per email.