I don't really understand this statements. Can you state this with your own words to make easier to understand?
No universal is a particular substance, numerically one; for if this were the case, then it would follow that Socrates is a universal; for there is no good reason, why one substance should be a universal rather than another. Therefore no particular substance is a universal; every substance is numerically one and a particular. For every substance is either one thing and not many or it is many things.
What William of Ockham is saying here is that only individuals exist. There is no concept that exists that is made up of more than one individual. Ockham, unlike Plato, for example, does not believe in the existence of "forms" that include multiple individuals. Just before the passage you quote, he says that there is no "universal" thing that exists "outside the mind." This is what he is trying to say in this passage.
To paraphrase him:
You can't have a universal concept that is made up of lots of different individuals. You can't, for example, have a whole lot of things, all of which are called Socrates. Socrates was just one man. That is why there is no particular (individual) thing that is also universal (made up of many things).