Children investigate number before, and counting forwards is adding one each time and therefore the number after is one more, and number after, and counting backwards is taking away one each time and therefore the number before is one fewer / less simultaneously. This ensures children develop deep relational understanding that they are able to apply to further concept levels.
After children have developed their capacity to count with one-to-one correspondence, first to 10 then to 20, they begin to develop their understanding that counting forwards is adding 1 each time.