Rectangle being divided into equal columns

If you have Illustrator, you could use the Rectangular Grid Tool to make a rectangle that is divided into equal spaces. Just select the tool, click on the artboard and type in the numbers that you need.

You can then copy the rectangle from Illustrator and paste it into Photoshop. Choose to paste as a smart object so that you can easily resize it and use it as your guide.

It's easy in Illustrator, too.

In Photoshop, I would add columns guides to make columnns and rows. Set the margins to zero. If you need to actually divide the rectangle itself, create a rectangular marquee  and cut each section to its own layer. These instructions are brief, so post back if they are too brief.

Someone else may have another idea, possibly with scripting.

Jane