Ich vermute daß das Besucher sind, die im betreffenden Zeitraum nur ein einziges Mal vorbeikommen (die restlichen 1200 Zugriffe stammen von 1-600 Mehrfach-Besuchern).

Note that unique in English is only an adjective, not a noun. "Uniques" as a noun is not standard English.

Having said that, an English speaker would probably be able to parse the sentence easily: The website got 2000 visits monthly, including 800 [visits] which were unique. An English speaker would not necessarily understand the deeper meaning, which is that the remaining 1200 are repeat visits. In other words: During the month there were 800 people who visited the website at least once; some of them visited it more than once, so that the total number of visits was 2000.

"my website has 2000 monthly visits with 800 uniques."

Its kind of slang-y usage, so I tried rewriting it more formally for you:

My website gets 2000 hits a month.

My website gets 2000 hits a month, and 800 are unique visitors.

I get 2000 hits/month, and 800 are unique.

Any native speaker that is used to computer-geek-speak would know what you meant. I'd probably say something like: I get 2000 hits/month, of which 800 are unique. If I was being super formal.

Falls die deutsche Übersetzung gebraucht wird: mein ISP verwendet "eindeutige Besucher", was besseres weiß ich auch nicht, da man im Deutschen dazu tendiert, die englischen Ausdrücke zu übernehmen, wenn's um Computer geht.