What is the theme for the quote "There's 24 of us, Gale, and only one comes out"?
If you look at the enotes page for themes in The Hunger Games, one of the book's themes is survival. The quote listed in this question is a good supporting quote for that theme.
The Hunger Games arena is a dangerous place to be. Twenty-four contestants will be pitted against each other in a fight to the death. The last person left alive wins. The odds of survival are not high for Katniss. She doesn't come from a district that trains tributes. She's not especially aggressive. She's not physically big or strong, either. The quote in the question bluntly points out that Katniss is not likely to be the one person who survives and comes out of the arena. She'll go in as one of the 24, but she's quite certain she'll die as one of the 23.