The first chips made on TSMC's 5nm process will be for Apple, the A14 SoC to be more specific. This SoC will end up in the next iPhones, which are expected to hit retail stores in September if all goes well.
TSMC's 5nm capacity is already fully booked. About two-thirds goes to Apple and the remainder goes to an unknown client (or clients).
Yields are said to be very good. In December 2019, TSMC said they were seeing yields upwards of 80 percent for 5nm EUV.