I don't count shots with numbers, just with pops, if I need to fire six shots during my walkthrough I hear pop, pop, pop, pop, pop, pop in my head to ingrain the rhythm; I think the words one, two, three, four, five, six require too much though. Then again I've never paid attention to how fast my splits nor practiced shooting them faster.
I GM I've shot with a few times pointed out that the best shooters didn't have fastest splits, but their splits were almost the same as their transitions. They shot with a smooth rhythm: pop, pop, pop, pop instead of the guys who were more concerned with splits: plap-plap... Plap-plap.