He doesn't care if everyone thinks Umi is number two. She's always number one in Maki's heart.
I think it's in the narration episode where Maki, Yuu, and Nina peeked on Umi.
Being number 2 did make Umi popular in its own way. Yuu, as number 1, is like an idol you think is cute but never seriously believe you'll be dating, as she is in different level. For some people, that makes Umi felt more human and approachable (and attainable).
Like, Yuu is someone you felt enough just by looking and sharing stories with friends about how cute she is; while Umi is someone you actually want to know in person.