I recently purchased a TENS unit for my wife. It came with one pad holder, however you can use up to 6 pads at a time to the specific unit I purchased.

Since a holder can only hold two pads, when they aren't in use (one on the front, one on the back), obviously that's not enough. Pad holders sell for an outrageous price on amazon, so I modeled one, and here it is.

Picture of an "official" that I used as a size reference and the actual print included.

Example of pads that will fit onto this:
https://smile.amazon.com/gp/product/B0181CS9TW/