I often hear the guards or even peasants say some slurs about me , but one asked a question thats been bugging me...
"What Happens to Witchers once all the basilisk etc monsters are all gone" (not exact words)

Is Geralt the last generation of Witchers? Or is there some Secret Witcher School that was hidden from the world and
is somehow making more Witchers because they were isolated and think the World still needs taming or something?

In the first game, there was a time when Geralt asked something of the sort to Zoltan. I don't think that there is any other witcher school, but who knows... Maybe Letho can find a way to reopen the school of the viper in one of the endings (I'm not saying that it's mentioned, because Letho isn't mentioned in the epilogue). Then, there are witchers who become hired killers, and others who try to live their lives as normal people. But I don't think that witchers are still needed, not when you have a whole unit of humans learning alchemy and reading books about dark magic, etc. (witch hunters).

Technically not like a Witcher is required to create a new Witcher, you take some hardcore drugs (Trial of the Grasses), and consume mutagenic compounds in order to become super-human. A part of the plot from Witcher 1 actually involves an organization stealing the mutagens/formulas from Kaer Morhen and trying to make a super human army with them.

The only reason for having another Witcher around for is the knowledge of which compounds (which could be passed on to anyone) you need to consume.

Granted not very many survive that process, but if you do your natural life span increases by like 100 years or more.

Other than that, they are then trained in the usage of Signs which I assume any mage or Witcher could teach them. (Don't think if it was ever explained if Mages have the ability to do Signs as well, but you'd assume power hungry mages would love the power of Axii.) Signs are also the reason Witchers prefer to not use 2 handed weapons (to have a hand free to cast Signs) Might even be possible to learn how to Sign from books alone.

And Combat tactics are taught as well, which yet again, anyone with fighting knowledge could teach.

The whole concept of Geralt and company being the last of the Witchers is mainly a plot device to add urgency to the plot. They also seem to magically make Witchers appear during the plot whenever they feel like it, which is kind of odd seeing as there are so few of them you'd think they'd all know each other.