Counting is unreliable so I wouldn't recommend it. I used to use fingers, by putting fingers on pieces I've traced and just mentally keeping track of leftover targets if I run out of fingers. As I got faster and more experienced, I stopped having to do this as I developed a sense for knowing if there are pieces left/where they are and I'm now able to subconsciously track targets without fingers. Another technique you could try is something I use sometimes in big BLD, where you just go through each target individually and check if it's in your memo.