Hi Alan, welcome to the community, hope you enjoy your stay. [excited]
I'd like to suggest you try the "Contact Sheet III" app within the "FILE>Automate" menu... but that may not actually prove to do what you need it to. Specifically because i think the images will get at least a 1-2 pixel paddig around them; so they won't be completely snug to each other.
But you may as well give 'er a go anywho!
Below are the settings you'd need to use.
Change the output resolution if you need to. If not, just use the default 72. And you'll need to name your images in a sequential order --
using letters or numbers.
Click the "Browse" button and find the folder containing the source images.
Once you're setup, click OK and PS will begin the procedure.
Let us know if this worked for you.
If not, you may have to resort to trying to create an Action/Droplet for what you need. But that's a whole 'nother story... [confused]