One more means the number that comes next when you are counting. One less means the number that comes before when you are counting.
One more than 2 is 3
One less than 3 is 2
One more than 5 is 6.
One less than 8 is 7.
How many pears does the dog have ?
The cat has 8 pears.
The dog has one more pear than the cat. One more than 8 is 9.
So, the dog has 9 pears.