You don’t lose fat cells when you lose weight, the fat cells simply become smaller.
In people who have an excess number of fat cells, the fat cells may be even smaller than normal when they get down to a normal weight.
That may be one reason it is so easy to regain weight.
The average adult has 40 to 50 billion fat cells, and some very obese people can have 120 billion or more.
The best advice is to avoid excessive weight gain at any age, because it can cause new fat cells to be made.
It may be particularly easy for this to happen before the age of sixteen.