Überzahl literally translates to "over-number," meaning a majority or an overwhelming number of something. It's a less common word than simply saying "many people" and carries a slightly more formal, weighty tone.
In the song, Madeline Juno sings "Glückliche Menschen in Überzahl" ("Happy people in the majority"), using this word to emphasize her feeling of being an outsider, surrounded by an overwhelming number of people who seem happier than her.
Have you ever been out with friends, or at a party, and felt completely out of place? Madeline Juno totally gets it in "Normal Fühlen" [Feel Normal]. She describes being surrounded by "Glückliche Menschen in Überzahl" [happy people in the majority] and feeling like it's a "Schlechter Moment um grad da zu sein" [bad moment to be there]. It's that classic feeling of FOMO, where she didn't want to go out but has "immer Angst, was zu verpassen" [always afraid of missing something].
So, what does she do? She goes through the motions, dancing to songs she doesn't "gar nicht fühle" [feel at all], talking to guys she doesn't "gar nichts will" [want anything from], and taking a shot "weil jeder hier einen nimmt" [because everyone here is taking one]. All she wants is to "mich doch nur normal fühlen" [just feel normal]. She's stuck in her head, thinking "Ich mache mir 'nen Kopf und es fällt keinem auf" [I'm worrying and no one notices], wondering if she's "das Problem" [the problem]. It's a really relatable song about trying to fit in when you just don't feel it.