Usually:

Horizontal if they are singles.

Vertical I they are duplex, because they are square anyway.

Vertical if they are triple, because they are rectangular anyway so read horizontalish.

Usually you do not have all three mixed. If you have a mix you can decide whether to do the singles horizontal.

I like horizontal outlets anyplace, but it doesn't bother me if the switches are vertical necessarily, it just depends on if I am trying to fit them in a tile layout and then only the singles need to be determined anyway.

I have one set of switches at the end of the backsplash run which is vertical. The other two switches are mounted under the cabs in low-profile boxes. It works well for me.

Thank you! Never thought of differences based on the number but that makes total sense. I believe we'll have a triple, so I should be set. It's all in the details!

The light switches we will have in the backsplash area are going to be in shallow boxes mounted under the upper cabinets, so we won't see them.