noun which may take the genitive case particle 'no', noun
one of Japan's foremost; leading in Japan
Used attributively with の to describe something that ranks among the top in Japan. Often found in formal or written contexts.
He is one of Japan's foremost pianists.
This is one of Japan's leading tourist destinations.
有数 alone means 'prominent' or 'leading' and can be used for any country or region, while 日本有数 specifically means 'one of the foremost in Japan'.
Compound of 日本 (Japan) and 有数 (prominent, leading).