you don't think "one" every time you do something once. just like that you can train to not think "one-two" and not to think "one-two three-four" and so on. gets harder the larger the number, but it's a matter of practice.
Like bars in music? Like I can put on the metronome to, say, tic-toc-toc-toc (different sound on 1 vs 2/3/4) and will intuitively know when I've hit 2, 4, 8, 16, 32 bars. I just feel it. I might be able to use this method for counting, just hearing clicks, and count to 16 bars or something, while doing whatever else, even though I am like Feynman in not being able to talk out loud while counting.