How can I retrieve my Administrator name.

See these articles OS X El Capitan: If you can’t remember your user name

macOS Sierra: If you’re asked for an administrator name and password

A very easy method is open terminal application through spotlight and execute a command whoami

The word is single and no space between alphabets , it will immediately show the admin name .

I used very convenient method to open terminal application , by just typing terminal.app in spotlight .

Then in the terminal window type the command whoami

it will always show the admin name .