This is done with css... you have to create a div and assign it a width large enough to hold only the amount of thumbnails you want contained in a row.
I would recommend using the testing theme and play around with that since that is probably the closest to what you are trying to do.
I noticed that div tag used to display thumbnails in an album is called "images" and I did not find it in my sterile-light css file, so I just added it with a parameter "width" to suit my needs. It works.
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I would recommend using the testing theme and play around with that since that is probably the closest to what you are trying to do.