to set the number of images that a WP gallery displays
Since the total images in Native WP gallery is set by users upon creating the gallery, so I suppose you mean the number of image column.
We can do that by using “columns” option. The default when not specified is 3, we can change this putting our own number.
Suppose we have this, which will display 3 columns
[gallery ids="729,732,731,720"]
We can do this to display them in 4 columns
[gallery ids="729,732,731,720" columns="4"]
offer navigation to the next image(s) in the gallery
To do that we need to use some lightbox JS, look for a plugin that says “works with Native WP Gallery”. Since you already have Jetpack installed, try activate the image Carousel module and play around with it.