How can I show the total number of photo's in a gallery?...
If you set-up and assign your albums to a "Gallery" Category, make a note of the ID number assigned by WordPress for the chosen Category.
If, say, your Gallery Category turns out to have an ID of 10, you need to create a template file called "category-10.php" in your theme. You use that file to list your albums to suit your particular theme design.
Within the template file, in the code loop that displays each album title and description etc., simply include the line:
This album contains <?php echo $count ?> images.
If you use a search engine on the web, you'll find that people have written tutorials on how to achieve various results with the inbuilt WordPress gallery functions. There is also some info to be found in the official WordPress Codex.