Without a Misery —
Others — whose instant's wanting —
Would be Eternity —
The last — a scanty Number —
'Twould scarcely fill a Two —
The first — a Gnat's Horizon
Could easily outgrow —
F574 (1863) J372
Dickinson is making a comparison between the people she cannot do without, no more than two, to those she could miss without any misery. While I want to read the poem as saying that there are oodles and oodles of folks she could do without, a close reading is less clear.
The last two lines are a bit ambiguous: could the number of lives missed "Without a Misery" "easily outgrow" the gnat's horizon? If so, then the expendable lives could easily outgrow a radius of, say, a couple of feet (the scope of a bumblebee, according to one source). That's a small base to start with and although the phrasing doesn't limit the number of people, it doesn't necessarily follow that it includes almost everyone the poet knows. It might as easily mean ten people as a hundred.
If it is the gnat's horizon that could easily outgrow the number of expendable lives, then there really are only a few people she could miss without misery. If this is the case, the poem is saying, "there are two people I can't do without and a number of folks I don't give a fig about".
Of the two readings, I prefer the former. It seems a better contrast to the two essential people, and contrast seems to be what this poem is about. But smitten as I am with the phrase "Gnat's Horizon" (which would be great fun to work into a conversation), it just doesn't establish a dramatic contrast. It doesn't allow us to conclude that Dickinson doesn't care for many people.
Instead I think she is emphasizing how much she cares for two people. It's a bit of hyperbole, but she claims that every moment she misses one of them would be an "Eternity". That's quite a burden for the lucky two.