From the poem itself, it is not evident that Dickinson is trying to help anyone specific. Therefore, it is possible that she's saying that if just one person is impacted by her poetry, she "has not lived in vain."
It's important to take a step back here, however. When reading poetry, we must make sure that we do not just assume that the poet is the speaker of the poem. The speaker of a poem is similar to the narrator of a story. That being said, I think we can make the case that Dickinson herself, is speaking here.
If we make that choice, we can tell by her word choice, or diction, that she doesn't mean anyone specific. For instance, in the first line, she says, "If I can stop one heart from breaking." She uses the word "one," rather than "your" or someone's name. She also uses "heart" and "life" as what she's trying to save, which are very general terms to the concept of humanity. She does however, become specific in talking about the robin. In that light, we can further our case that she's being general about humanity.