It basically means ‘that’. It’s a little more formal-sounding I would have stayed. You should have known that I would have stayed. (Fully repeated, for drama or emphasis) I would have stayed. You should have known that.(standard) I would have stayed. You should have known as much. (Higher register)

Great question - it is actually an idiom, which means, "the thing I expected, hoped or feared would happen". For instance you can say, "I knew he'd be able to learn Italian, I said as much at the time" (i.e. "I said I expected him to be able to learn Italian at the time") It also has a slightly different use in sentences like, "Please have me to stay next week, you know I'd do as much for you", where it means "the same".

Referring to the sentence above **as many as