A SmugMug gallery is a gallery of images which you upload to your SmugMug account.
You can can have as many galleries within your SmugMug plan as you like, there is no limit.
There is also no limit to how many photos you can upload to SmugMug.
Also a gallery can have just one photo in it, or thousands.
All the galleries can be shared separately to Facebook or via email. Or the galleries can be stored privately.
The choice is yours how you would like to store, share and organise your photos.