"what can I do, that from a gallery with 95 pictures only the first 3 are displayed as a thumbnail and not all 95 pictures"
Please keep in mind that Envira Gallery is a gallery plugin. It will display images that you have assigned to the gallery that you have created. If you want to display 3 thumbnails, you only have to assign 3 images only. With Envira Gallery settings, you can only limit the number of columns but not the number of images because it the first place, the images were assigned or added as one of the gallery images.
Hope this make sense.