"He has several suits."
Translation:Han har flera kostymer.
The construct "en/ett ..., flera ..." is common for showing the forms of a noun in Swedish since it will actually give you all the information you need on the noun and its different forms. As far as I recall from school at least our English books used similar constructs with "a ..., many ..." when introducing new words that didn't have the standard form of just adding an -s. Maybe that is why the other translation is like that.
Also as with everything there may be slight context variation.
I another possible (and more precise) translation for several would be "åtskilliga" but that has a lot more formal tone in it than flera.
Declension, usually. You can find some great info e.g. here: https://www.thelocal.se/blogs/theswedishteacher/2010/07/28/plural-endings/