If you see one rat inside your house then there likely are some other rats living either underneath your house, in your attic, basement etc.
But you can indeed have just one rat coming into your house looking for food while other rats may be in other locations searching for food.
If you have a rat you'll usually notice that the rats come out during the night when you sleep because rats mostly sleep during the day and come out during the night.
At night most people are also asleep so it's safer for the rat to come out during the night when nobody is awake.
But when I have rats in my house the noise they make knocking things over etc wake me up from my sleep.
I've used both rat poison as well as live rat trap cages to trap the rats in and then kill the rat later the next day.
Never release the rat back outside without killing the rat first because it will then come back into your house again.
Rats can cause severe damage to your home including the electrical wiring as well as carry diseases that can be passed onto humans if the rat were to bite you which they can do so.
My younger brother when he was 2 years old was bitten by a rat who climbed into his bed.
He got sick from the rat bite but he was okay after taking some medicine and he's doing well now.