Using count() in Relational Algebra

Sell (Bar, Beer)

I want a table in which the first column has the name of the beer and the second has the total number of bars which sell that beer, but I have no idea how to use count () in the right way to obtain this.

Could it be Beer G count(Bar) as count_bar (Sell) ?

And later that, how could I set a table with the best-selling beer (or the best-selling beers)?