EDIT: 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.