Sorry for being unclear.
I mean this one " The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water, and other necessities available in the environment."

How many is it for all humans on the earth?
How many humans can earth provide food,oxygen,water,etc for them?
Can we calculate it?

there is no precise way to measure it in this context largely because there are very many variables - many of which are contentious to put it mildly. a google search brings up a good few sites that discuss this broadly - as you will see the numbers and extrapolations vary wildly!