A researcher in animal cognition said at a lecture I saw once: every time we look closer, we see more and more evidence of intelligent behavior in tinier and tinier creatures, behaviors we once assumed were limited to humans. He studied insects: bacteria have been shown to display “learning” behaviors.

How many beings are sapient? Wouldn’t be shocked if Future Being told me it was all of them

To my knowledge, the sapient population is the human population of Earth.

However, there are numerous sentient species that could conceivably be "uplifted" in the next thousand years. Plus, I consider it more likely than not that extraterrestrials do exist and that there's likely another sapient species out there, though they are unlikely to be in our general volume of space