The: you’re talking about a definite item, which is why "the" is called a definite article. Of course it only makes sense if both you and your listeners know which item you mean. If I commanded you, “Give me the money,” you would rightfully ask, “What money? I don’t owe you any money.”
The house on that corner once belonged to Charles Dickens.
The weather is very pleasant today.
You can use "the" the second time you refer to something, even if you used "a" the first time. We know what you’re referring to, because you just told us. You can do this, but I wouldn’t recommend it. Read these examples – repeating the noun might sound monotonous.
We visited a palace on our vacation. The palace was built in 1546.
We also went to a concert. The concert was too loud for me.
A: you use this when you’re not necessarily referring to a specific thing, such as a nail, any nail. It’s called an indefinite article, because you’re not being definite or particular. There are many nails in this big world.
She owns a cat.
I work on a golf course.
In your case
What you use depends on the contextual knowledge of whomever you are speaking to. If you are talking to someone who would know what number of people you were referring to, you could use "the". However, if this is the first time you are bringing up this group of people, and you don't have any other way of identifying them specifically from a (if relevant) different number of people who may be close to the owners, then you would use "a".
EDIT: Just want to say that usually you'd say "a number of people", not "the number of people". Though you could grammatically make that work, and it would make sense if you really meant it, it would sound odd and it doesn't mean what you most likely want it to mean.