Currently it is not known. Earth is the only known planet to hold life so far. It also depends at what point you consider a living thing to be sentient.
Yes. It might be possible that there are countless sentient species in the universe, but we do not know for now.
Despite much debate among people and scientist, there is no official proof that there are countless semi-sentient species in the universe.
In my opinion, yes. There are at least 10,000 trillion sentient species (100,000 per galaxy) in the observable universe.
Though some scientist say there is a possibility that there are countless non-sentient species in the universe, there is no concrete proof right now to be sure.
It isn't currently known whether there is even a single species outside Earth - sentient or otherwise. The number you mention has been made up by somebody.
315,000.
1,260,000.
1,260,000.
1,008,000.
201,600.
26,460.
56,700.