When quoting plays, kindly give the Act and Scene numbers so people can find the passage more easily. There's usually going to be some context around it.
In this case, there's a pun and you only told the first part of it. The full passage ends with the clown saying Truly, I thought there had been one number more, because they say ''Od's nouns'.
Now, this isn't going to be funny, because explaining jokes seldom works within a language and we're translating another one here, but:
Evans is emphasizing his own poor education by saying "how many numbers is" instead of "are". The word number here isn't using the common meaning but the grammatical one, where you have singular and plural.
The correct answer is "two" and that's what William says.
The joker notes it should be three (It is in Greek and for some places in English where we have a dual case) since people say Od's 'ouns (i.e., nouns are odd, not even, in number). The pun is on G‑d's wounds, which was a mild oath in Shakespeare's time in reference to Jesus's wounds from the nails on His cross.
It's something like a little girl being asked how she would cross the river and answering dammit because that's what dad is always saying. It's right and cute and a little naughty and somewhat embarrassing to any adults the kids have been listening to.