Basically certain words use a different counter, and it changes how you would say 1, 2, 3 etc. so there is a counter for big living things, small living things, flat things, round things, money etc. A lot of the numbers are similar with slight changes in pronunciation.
So instead of ichi for 1, it might be ippon, ittou, issoku etc. if you know it's a counter you can usually tell what the number is from the start of the word. Though there are exceptions like hitotsu, which is also one. Though if they use the "Chinese" characters you can know what it means when reading it without knowing how it's actually pronounced.