How many bitcoins are created in each block reward?

How to get close to this number

Work back from the halvings. The explanation gives the starting value of fifty in 2009 and a halving roughly every four years. Fifteen years is close to four halvings, so fifty halved four times gives a bit over three — and the arithmetic is more reliable than recall because the schedule is fixed.

That fixed schedule is the design point. The issuance rate is written into the protocol and nobody can change it without consensus, which makes the supply curve knowable decades in advance. Almost no other asset has a publicly verifiable, pre-committed issuance schedule running to the next century.

The endpoint in the explanation is the structurally significant part: rewards reach zero around 2140, at which point exactly twenty-one million will exist. A supply cap enforced by arithmetic rather than by an institution is the entire economic proposition, and the block reward is the mechanism that delivers it.