EDIT: May 2021August 2022
Total ETH supply: ~119,300,000 ETH (as of Q2 2022)
With Proof of Stake, the annual issuance rate will be ~0.49% (with 13 million ETH staked)).
This is a daily issuance of ~1,600 ETH.
At an average gas price of at least 16 gwei, at least 1,600 ETH is burned every day, which effectively brings net ETH inflation to zero or less post-merge.
Source: https://ethereum.org/en/upgrades/merge/issuance
Written May 2021
The total supply of ether (ETH) will vary because EIP 1559 will burn ETH, and Proof of Stake replacing Proof of Work will reduce the amount of ETH created per block.
If more ETH is burned than created, it would actually decrease the total supply of ETH.
Justin Drake, Ethereum Researcher, discusses the supply in Modeling Ultra Sound Money. His spreadsheet from his tweet:
The total supply of ether on April 25 2021 is 115,620,035 ETH.
Very old answer below.
Other answers have covered the topic well and 2 things to clarify:
The current issuance rate is known: 5 ETH every ~15 seconds (Bitcoin is currently 25 BTC every ~10mins).
With any changes to Ethereum, such as the transition to PoS, the issuance rate is guaranteed to not increase. But it may decrease, possibly to zero or something much less than 5 ETH, when the transition to PoS happens.