How many Siacoins will be in existence?

Siacoin, unlike Bitcoin, has built-in inflation.

The initial block reward was 300,000 siacoins. Each block reward after that is one siacoin smaller than the previous block reward (299,999, then 299,998, and so on). When the block reward reaches 30,000 (at block 270,000), all remaining blocks will give a 30,000 siacoin reward.