I have 50 photos with different size from 50 KB to 70 KB. I want all photos tobe in 60-70 KB size without changing the dimensions, dpi, or bit depth.
Using compression you can easily force output size to be constant.
For example, you can easily use IrfanView Batch Conversion functionality to just resave all your photos at once with a fixed size: just set conversion options to 100% quality and enable Set File Size specifying the wanted KB size.
Since you specify Photoshop this can be done directly in Photoshop using the Export for Web panel. Since you're doing it many times you'll probably want to setup either an Action, Batch or Droplet.
File > Export > Save For Web (Legacy) [or just Save for Web if on a Pre-CC version]
Then can open the additional options within the dialog to see Optimize to File Size:
Which pulls up a simple prompt asking you what size to make it:
Turn this into an Action and then either Batch or Droplet it to automate the entire process.