"The restaurant has several cooks."
Translation:Restaurangen har flera kockar.
Okay, so here's where it gets tricky. I'm going to try to explain this as clear as possible.
Used as indefinite number of things by itself, without any kind of comparison at all and has a meaning of many or numerous. Some examples:
Det är många som börjat lära sig svenska på Duolingo.
There are many people who have started learning Swedish on Duolingo.
Vi var inte så många som tyckte om filmen.
We weren't that many who enjoyed the movie.
Used as some kind of (vague) comparison, meaning more than once or several. Some examples:
Det är flera som börjat lära sig svenska på Duolingo.
There are several people who have started learning Swedish on Duolingo.
Var var flera som inte tyckte om filmen.
We were more than one who didn't enjoy the movie.
There are sentences though, where flera is being used and it could come across as both more than once and many in the translation, for example:
Det finns inte bara ett, utan flera problem.
There's not only one, but many/several problems.
I wouldn't say the mouseover hint is wrong, but if you stick with the concept that I presented, you'll be more likely to say things correctly. (: