Maybe so, although CICO (calories in, calories out) is a little oversimplified. In reality not all calories are equally satiating, nor are all calories equally digestible.
Additionally, measuring calories in food is done in a purely chemical fashion - the food is burned and the amount of heat energy released is measured. This of course does not actually measure how much energy your body is actually able to absorb (for example, dietary fiber burns just as well as refined sugar, but isnt absorbed the same in the body).
If you want a upper limit it's around 20000 calories. That's what eat people who go to the artic / antartic.
Honestly i think its WAY LESS oversimplified that people say it is. True you never know what you really ate, but if you note your average calories for a month, and then do +20% or -20% calories with the same type of food and same lifestyle you will get the exact weight loss / gain predicted from regressing on the first month.
Additionally, measuring calories in food is done in a purely chemical fashion - the food is burned and the amount of heat energy released is measured. This of course does not actually measure how much energy your body is actually able to absorb (for example, dietary fiber burns just as well as refined sugar, but isnt absorbed the same in the body).