Astra at Sandia Labs was the first Arm peta-scale supercomputer, and the first on the Top500. It debuted in 2018.
Fugaku is the fastest Arm supercomputer, taking the #1 spot on the Top500 in 2020. It is currently #4.
All NVIDIA Grace-Hopper systems will be Arm. There is one in the top 10 already, Alps at the Swiss CSCS, at #6. There are four more in the top 100.
Apparently I am super wrong here. ARM has made serious inroads here as well.
I think that ARM's willingness to allow their IP to be customized for their clients needs has really given them a lot of competitive advantages.
Right now RISC-V is ultra slow in all implementations I've seen. Like 30x slower than a top of the line Apple Mx series CPU. Maybe there is a high performing RISC-V chip out there but I haven't yet run into one.
RISC-V benchmarks: https://browser.geekbench.com/search?q=RISC-V. Compare to an Apple M4 benchmark: https://browser.geekbench.com/v6/cpu/8224953
That said, RISC-V is good for embedded applications where raw performance isn't a factor. I think no other markets are yet accessible to RISC-V chips until their performance massively improves.
Sometimes useful to see how cpu architectures grow and then get crowded out by the next processor family -- at least among supercomputers.