Adding multiple slides to slider

Didn’t really get.
You have 4 images. You can put all 4 in one slider.
Doesn’t work for you?
https://www.themesandco.com/creating-a-slider-with-the-customizr-wordpress-theme/

No, it doesn’t. It only gives me 2. When I add a 3rd, it just replaces one of the existing 2 with the new slider.

I’m sorry really can’t figure out what could be the issue.
It’s something that simply doesn’t happen to me, I cannot reproduce it, also this is the first time I read about it in this forum.
But what happens? As soon as you select a slider to add your new slide you see that this last one replaces one (first or second?) of the already existing slides? Or is something that happens after ..?