It looks like this is kind of complicated, as the entry on Wiktionary indicates "many" as a possible translation for flera. It seems that the word in Swedish which officially means many ( många ) is linguistically considered a noun; but it also seems that in common use many Swedes DO use it interchangeably with flera. I suppose it's like English speakers don't always know the difference between less and fewer, and the language lessons are trying to teach the -correct- way to say it rather than the common way.
I was wondering about this too... "several" seems an odd choice of a word to teach here since it's not that natural a word to choose for an English sentence like this. Some would be more normal although I guess in use is pretty interchangeable with several.
This keeps making me think of Blackadder trying to teach math to Baldrick: "I have two beans, and then I add two more beans. What does that make?" "Some beans."
So I wrote, "one cat, more cats" as the previous exercise stated "flera" to mean "more/several." When I used "more cats" it was marked incorrect and said it should say "several cats." Is there a way to know when to use "flera" to mean "more" vs when to use it to mean "several"?
"Många" is usually used to express size of a group, while "flera" when adding to a group. This goes right against this example, to make your life complicated, as it is more of a phrase. "I want more cats" would be "Jag vill ha flera katter", and "I want many/several cats" would be "Jag bill ha många katter".
To confuse you even further, sometimes Swedes use the word "mera" interchangeably with "flera". Though "flera"should be for an amount you can count: "jag vill ha flera äpplen" - "I want more apples", "jag vill ha mera vatten" - "I want more water".