Are många and flera interchangeable?
Or are there like certain situations where you use one and other situations where you use the other.
another question, is det pronounced as de or with the t because memrise says it with the t but duolingo says de and some forvo pronunciations say both.
My native Swede friend says this:
"Nnnot quite. Flera doesn't feel like it's describing something quite as numerous as många. All flera really means is more than one. Många means it's probably not just two or three, but many of them."
He also mentioned "några" means "a few" and "ett par" is "a couple/pair" and flera is more than a few but not a whole lot.
So go with this: ett par < några < flera < många
This is what SAOL (the dictionary of the Swedish academy) says:
flera = många
fler (or flera) = a bigger amount of (used for comparison): Jag har fler hundar än du
My feeling is also that "flera" is less than "många", but that "flera" has to be more than two.
About the "t" in "det": Normally, it is not pronounced. If you do, you will sound very formal.