Counting as declarer is a critical skill. We've often heard about counting winners or losers, but most players aren't in the habit of doing it — and doing it repeatedly as the hand develops. Applied correctly, it's a very powerful skill and the basis for any good declarer planning.
Counting is usually very simple — it's about looking at basic suit holdings and counting to small numbers, for example up to 7 or 10. Or, when counting losers, it's often just making sure there aren't 4 losers when you're in 4♠. It sounds basic and pointless, but as you do it, an amazing skill gets unlocked — you'll be in the top 1–5% of declarers!