Let me say a few words about my translation of the phrase des Nombres et des Êtres as from Formulas and Souls. Baudelaire obviously chose the word êtres in part because it rhymes with fenêtres (windows), and scholars are not entirely sure what he meant by it. In French, être means being, and translators often translate this phrase literally, as Numbers and Beings. For example, see these translations. (I’ve noticed that translators tend to use literal translations for anything they don’t understand.)

My interpretation is that the word Nombres references Pascal, who was mentioned back in the first line. Furthermore, I think that by Êtres, Baudelaire also meant to reference Pascal, in particular, his other calling besides mathematics: religion. If this is the right interpretation, then formulas and souls is just as good a translation as numbers and beings. And if it’s the wrong interpretation, souls is still a fairly good synonym for beings. So it works either way (and also rhymes and fits the meter).

One more point: My interpretation of the last line is a little different from most translators'. Most of them seem to see never leaving numbers and beings as a bad thing. I think the speaker thinks it's a good thing ... numbers and beings (i.e., math and religion) provide an escape from the real world, which for him contains horrible things like nightmares and vertigo. I have found one other translation where the translator appears to have the same interpretation.

Finally, if it hasn’t occurred to you already, let me point out that both nightmares and windows are “big holes filled with vague dread leading who knows where,” i.e., abysses. (Windows, in particular, for people contemplating suicide by jumping out them.)