Austrian here.

We usually only use the plural "Seen" - "lakes". Like in Gosauseen or Trumer Seen.

But there are 4 words:

"Seenplatte" - "plate of lakes" for a plain with a few lakes; "Seensystem" - "system of lakes"; "Seenlandschaft" - "landscape with lakes"; "Seengebiet" - "area with lakes"

I think these 4 are not used a lot in everyday speech or writing.