After the events of The Witcher 3, how many witchers are left?
Needless to say, massive spoilers ensue.
Those are fixed and are left alive regardless of your choices during the game.
Some of those may be left depending on your choices in this or previous games:
Lambert, will survive the battle of Kaer Morhen only if Keira Metz is there as well.
Berengar, from the first game. You may choose to kill him in the 3rd chapter of the game; if you spare him, he will come back and fight for you against Azar Javed, where he may be killed.
Letho of Gulet
If you spared him in Witcher 2, he will make a cameo in Witcher 3 and maybe even come to Caer Morhen.
Gaetan, the witcher from the Cat School from the DLC quest Where the Cat and Wolf Play. You may chose to end his life, or to spare him. In the latter case he will give you directions to his stash, where you will find a letter for other Cat School witchers:
It's over. Soldiers have taken the school. They killed Axel and Cedric. As for Schrödinger, well, I can't say for sure - might be alive, might be dead. Bounties have been put on your head and mine. Avoid cities and the high roads. Don't attract any attention.
Jad Karadin, the guy Lambert has a grudge against. You may choose to spare him, or may not.
In one ending Geralt gifts her a silver witcher's sword, and though she has not passed the Trial of Herbs, afterwards they hunt together with Geralt (which is also seen in Curse of Crows comic).
This witcher was last seen alive, but his current whereabouts and status are unknown:
- Brehen, from the ending of Season of Storms
So far I count 2 at least, 9 at best. Note that this doesn't count any witchers that don't appear on screen and are only mentioned tangentially, like in letters or conversations - we know they exist, but there's simply not much to go on.
Note the answer above forgot about Vesemir, the older Witcher who put Geralt and Eskel through the Trials of Grasses as kids. He makes an appearance at the start of the third Witcher game, and throughout it, he can be seen.
Making his final appearance in the battle of Kaer Morhen. (Forgive me if I misspelled it I haven't read/played much Witcher recently)