There are likely so many different intelligent
species in the Halo Universe, and we've only
seen a very small percentage of them.
While preparing for the eventual activation
of the Halo Array, Forerunner Lifeworkers
catalogued at least 123 sapient species.
We've only seen a dozen or so of these
technology-capable species in Halo's lore.
Almost all of these have been in the games.
It's possible that many of these species did
not survive after being re-established on
their homeworlds after Halo's activation.
However, it is more likely that a bunch of
these sentient species have survived, but
simply haven't been yet.