We had a different thread talking about 18650 batteries. This is a follow-up to that.
The 18650 battery at most holds 3400mah.
The item listed shows a cell phone size device that is 50,000mah. If the size is accurate that would be 4 18650 batteries. At 2,500mah that would be 10,000mah. That means that 40,000mah is missing.
At 2,500mah, an accurate size would need to hold 20 18,650 batteries.
Keep the form factor of what you are buying, in mind, and see IF the mAh matching the advertised storage.