Several years ago, we went to the churchyard in rural North Yorkshire, where my mum is buried. I grew up a few metres from this same churchyard, lived there 20 years and never saw anything rat-related (presumably because in the 60s and 70s they still employed a sexton, or something, to take care of the graveyard).
The churchyard is on a very steep hill and surrounded by a 12 foot high stone wall so it wasn't til we started up the hill that we saw a sea of rats, literally swarming, around a certain part of the graveyard (where the newest burials are). I remember saying to my husband I wouldn't want to be buried there - you probably get eaten by rats... If I'd just buried a relative there, I'd have been very disturbed and upset. As it was we just stood and watched them seethe for a while. I did think of ringing the vicar to tell him to do summat about it but promptly forgot and anyway he is a bit of a dick.
But yes, boiling seas of rats are a thing. They didn't cover the whole churchyard (they couldn't have - it is the biggest one I know, attached to a church - almost the size of some municipal graveyards). But I'd have said there were a couple of hundred of em. Luckily for her, my mum is buried right the other end but my grandparents are around that part of the graveyard and probably were being carried away in the rats' mouths, as they were in the past year or two, grubbed up for more recent tenants (They died in 1939 and the mid 1950s, respectively, but in an unmarked grave so I reckon the vicar thought that part of the churchyard was Fair Game).
My niece was getting wed there around that time and I did wonder if the rats might turn up in the background of the picturesque wedding shots but no sign of them, on that day. That vicar skins you for nearly £1000 for a cursory half hour wedding service, (25 minutes of which was him panhandling the punters for money) so you'd think he'd have a vested interest in rat destruction.