How to turn 7 photos into one (Lightroom 6) (microscope) (Depth of field)

Hey Jack, it's part of the Photomerge command in Photoshop. I honestly don't remember which version it was introduced in. When you run it, you choose between stacking and panorama.

Lightroom 6 can not do focus stacking.  It can do HDR and Panoramas.  Hopefully focus stacking will be in a future upgrade.  I used a plugin called Lr/Enfuse (Google it) for my landscape focus stacking/exposure blending but since I upgraded to Lrcc I now use Photoshop.  Lr/Enfuse worked well for landscapes but I don't know how well it would work for microscope stacking.  There are several dedicated focus stacking programs that may be better.

I think you have been misinformed, or perhaps you have misunderstood. Stacking in Lightroom enables you to group photos into a stack so that they don't take up as much room in the filmstrip display or in the catalog grid view. But it is not a way to combine images into a single image. Lightroom has features to merge photos to create a panorama or to create an HDR image. If you highlight all of the images and then right click on one of them you will see a Photomerge option available and you can choose between panorama or HDR. But I don't know if that is going to provide the results you are looking for. I suspect you will have better success using Photoshop or some other pixel editing software that will allow you to stack your images using layers.