Xiaomi 20000mAh is the real capacity of the powerbank. But it is impossible this powerbank can provide all it's power to phone battery, taking into account losses in the system as well as the cables and connectors. As stated by Xiaomi company, this model can give around 12700mAh of output.
How to verify your Xiaomi power bank is original or fake ?
1 >> Find the authentication label on your Xiaomi box, scratch off its coating to obtain the 20-digit security code
2 >> go to this Xiaomi website: www.mi.com/verify/
3 >> Enter the code for verification it