how to count distinct strings in a column?

I have a column that contains strings (Client IDs for the visitors of my website), I want to create a Card visualization that counts the number of visitors, so I have to count the distinct ones only, not all the values on the column, for example if I have these values (regardless that those are not client IDs) on my column: Canada, USA, Egypt, Canada, USA, I want the card to contain a number 3 not 5.