Translation:I think that there are lots of sites to see.
I love Duolingo, but sometimes I find that the English translation is incorrect. In English, when we say there are "a number of things," it means there are a lot of things. The meaning in English is the same as in French. So "I think that there are a number of sites to see" should be correct, but Duolingo marks it wrong. This should be corrected.
I think they want you to translate "nombreux" as "numerous" or "many" or "lots of", because it doesn't seem to mean "a number of" (which I think is "un certain nombre de").
With Duo, I've found that it's easier to use the closest translation, instead of synonyms, or similar phrases.
The English phrase is needlessly confusing for language learners. "Sites to see" is correct, and you might say that to mean there are lots of physical places you want to see, but it sounds the same as "sights to see" ("sights" meaning "things to be seen"), which is a far more common expression.
The difference between "site" and "sight":
Also, according to my dictionary, the French word "site" means a "place of interest" while "sights" is "attractions de touristiques", so the correct English would be "sites" and not "sights".