Adding sequential numbers to psd images

Although variables are designed for creating multiple variations from one template file, theoretically you could use this feature, in combination with Actions to update multiple files. I dont think you are wrong in your approach, it may work and I applaud you for persevering :-)

Here's the tutorial I made. I hope it helps you figure it out. Please let me know how you get on; https://www.lynda.com/After-Effects-tutorials/Using-variables-Photoshop/123481/128893-4.html?lpk35=8189&utm_medium=ldc-partner&utm_source=CMPRC&utm_content=524&utm_campaign=CD14836&bid=524&aid=CD1483

If you have a subscription that includes After Effects you could us the Numbers Effect to do this. In After Effects, import the images as an image sequence. (Check force alphabetical order if they are not named sequentially). Drag the image sequence onto the new comp button and then select the sequence and apply the numbers effect. change the settings to whatever numbering system you need, making sure it's set to create a unique number for each frame. Then go to Composition menu > Add to Render Queue. Render the comp out as an image sequence.

I use Windows you a Mac it may be the differences in culture but I have problem following what you write.

"I have a folder of 44 psd images and would like to automate adding a number to each photo in the picture area (not change the filename)." 

Where do you want this number to be if not in the file name.  Does each PSD contain a single Image or a  layer you want identified with a number, do you want a text layer added?  What do you actually want to do.

"My attempts using variables isn't working - Photoshop will only add a number to the image open in Photoshop and not the other 43 images in the same folder. All images and text document are in the same folder."

Photoshop variables are are associated with layers in a template psd file and columns in a CSV file used to populate the Template with your images and text  the process creates new files would not update your 44 file in any way.  Photoshop variable and open image I do not get what you are writing.

"I have tried using a plain text document with only the number required to be added to each image. Also a text file with the filename of each image, tab or comma, and number required to be added to each photo - reversed order als, number, tab or comma followed by filename. I have tried opening all of the photos in Photoshop, instead of only having the first image containing a text layer with number 1. I can change the number on the first image open in Photoshop but nothing happens to the remaining 43 images. Is there a way to instruct Photoshop to process all 44 images - apply a sequential number into each image please?

This is not about the filenam"

You are clear its not about File name you do not want them change and it is also seems clear you want your 44 psd files updated in some way that involves sequential numbers.

What is not clear is what you do want.  It seems you may a text layer added to these PSD files.     IMO the only way to automate that process using Photoshop is with a Photoshop Script.   Which is way above your pay grade.  For if you knew Photoshop scripting you would not have needed to start this thread.

If all the PSD are all the same size and resolution, have the same layer structure the script may be quite simple. If not the programming involved could be quite complex.

IMO you should open each in Photoshop one at a time and add the number you want where you want and if a visible number in the image the size and location you want.  Its only 44 files.  If this will become a regular ongoing process you need to do.  You could think about automating the process.  However it will take a very long time to learn to Script Photoshop, weeks, months perhaps years. 

Use Bridge or LR. Select all the photos you want, and use batch rename, using the file's current name as part of the naming process.

It could be done with variables and data sets, but I question the efficency for such a small number of images. You might want to search for a script.