"There are many countries."
Translation:Det finns många länder.
Tbh I don't think anyone can totally clarify that, it's a recurring problem in the forums here, but a general rule of thumb is that är is for more temporary presence and finns is for more permanent existence. This is why det är en fluga i mjölken 'there is a fly in the milk' is preferred, but är wouldn't work in this sentence.
No, there are quite a few of them. All ett-words that end in a wovel take -n in the plural, for example.
What goes on here is a pattern that transcends grammatical gender; many words that end in -and get -änder in plural. Among them we find änder, händer, stränder, tänder (ducks, hands, beaches, teeth).