There are PHC-28S with 16kB RAM and others with 32kB.
It's 16kB RAM if 12431 Bytes free is displayed at start else it's 28815 Bytes.
Arkanoid program is located at 4000h-BFFFh. The RAM of MSXs 32K is located at 8000h-FFFFh, so it can't work without RAM expansion.