flat　 の意味は？

With new infections in Japan largely flat at a few hundred each day in the country of 126.5 million, things have been recovering.

With new infections in Japan (which are) largely flat at a few hundred each day in the country of 126.5 million, things have been recovering.

つまり、前半の付け足しの節の部分は、
New infections in Japan, are largely flat at a few hundred each day.
と言っています。

New infections are flat. ということですので、新規感染確認数が毎日数百件程度で「横ばい」だ、と言っています。