What? Are we subtracting the number of Bitcoin in circulation estimated every 4 years? Lol after several years then there will a thousands in circulation 

Anyway, you might want to ask the Bitcoin miners regarding your query, because basically Bitcoin block halving is cutting the block reward for the miners. So, that means, If a miner will earn 12.5BTC per block , after the halving it will decrease to just 6.25BTC per block.
Now, If the block halving is just a theory, Bitcoin should've been close to reaching the max cap of 21,000,000 by now.