I am unable to message more than one photo at a time on my s9+ which of course I find rather annoying. I swear I have been able to do this in the past. My crappy LG phone I use for my business will send 4 5 6 photos at a time.
Uhm - I've never had any issues with sending multiple images via Text.. I'm using Android Messages - But that shouldn't make any difference...Wondering if it might be an issue with your Carrier or your settings / texting app?