Each breath is not timed. I note the time I start and the time I finish and the number of breaths. If I am being strict, I stop meditating if I move. If not, a movement has nothing to do with the counting of breaths. Sometimes I will lose count. Say for example I am on 38 and I lose touch with consciousness enough that I forget I am exactly on number 38. When that happens I estimate what the exact number is supposed to be--I will know it isn't 5 and it isn't 70; my estimate will be pretty close to 38--and I carry on with a little error in my number.