Go ahead and guess for the Japanese.
23 so far.
24 if Kawauchi does it at the world half.
And for the US.
The difference basically is commitment. A US runner gets out of school doesn't get a great contract for a mediocre collegiate career and stops.
Dozens of Japanese runners get a full time job, utilize their degree and train their asses off until they get to their goal or nearly die trying.
It's like they all have the mentality of LR's own Eddy Lee!