I’m not sure I’d trust that site. Quoting a 1911 source for codicology resources isn’t the best scholarly practice. It also uses terms very loosely, and I’m not sure the writer knows what he is talking about (e.g., “vellum skin,” as far as I know, there is no such animal as a “vellum.")
A 240-page manuscript, like Voynich, would optimally consist of thirty 8-page quires, with each quire made from one sheet of vellum, and each cow producing some 3+ sheets. (I don’t know exactly how the Voynich is composed; it probably has more quires of various sizes.) It would take, roughly, ten cows to produce the thirty vellum sheets. (If it’s actually parchment, that’s many more sheep.) That’s a fair number of animals. It’s not absurdly prohibitive, but it’s not insignificant, and it’s more than most hoaxers would lay out on spec.
(Technically, vellum is cow skin and parchment is sheep skin, but it takes a DNA test to tell one from another. Despite the technical difference, “vellum” and “parchment” are interchangeable for practical purposes, both as terms and as physical objects. So when a manuscript is described as being “vellum,” there isn’t much confidence that it actually comes from a cow and not a sheep.)