So I downloaded a deck and want to see how long it is gonna take me to complete it an for that I need to see how many cards I haven’t seen a single time.
The cards you haven’t see yet are those with the state “new”. The related query (to be used in the browser search bar) is
is:new. Similarly, if you want to show only cards / notes belonging to a certain deck, use
deck:"My Deck" (adapt to the real name). Having both in the query will show the intersection of the two sets, that is, the cards you haven’t seen yet in a certain deck. The name of the window should indicate how many cards that is (something like “Browser (1 of N [notes/cards] selected)” where
N is the number of either notes or cards, depending on your browser mode.
Another way is to click on the deck, then click on Stats at the top.