Could geralt beat sekiro in a random encounter?

>for whatever reason most were wiped by each kingdom's population and ruler or whatever reason.
That's not the main reason, though certain Witchers have been victims due to getting wrapped up in politics, the main reason is that fighting Monsters is dangerous as fuck, and it's a high lethality job. There aren't many Witchers anymore because no schools are making new Witchers and haven't for a hundred years, and in the Witcher 3 we're at the point where no one really remembers how to perform the Trial of Grasses.

True, Geralt could kill Sekiro in barely 5 seconds

The schools and their residents were all murdered.
Kaer Morhem is in ruins and vesemir was the last of the actual teachers in there. Without the druids which all got pitchforked they can't do the trial.

Sekiro is just an immortal human. He has no real magic. Heck, he isn't even a true immortal. He's more of a zombie than anything. As long as you cut off his limbs, he ain't doing shit anymore.

Can you name the times he has lost?

No, they scattered and were picked off when Witchers picked up a more solitary life style as their numbers were reduced and Witchers made less frequent visits to their "Home" castles. Castles require extensive upkeep and repair that Witchers could not afford or effectively perform on their own for obvious reasons. I don't recall the trial requiring druids, though it does have a magical component, it can be performed by any mage, as I think Yen does it for Uma

Genichiro cut off his limb and look what happened.

No way to tell, since it's random

cucked satan as in ressurected? Sekiro does that all the time lol