"They are many."
Translation:Elles sont nombreuses.
As lemog said, you can say either "Ils sont nombreux" or "Elles sont nombreuses". Both translate into "They are many". You have to choose "Elles" when you talk about a group of female persons or objects only, and you have to choose "Ils" when you talk about a group consisting of at least one male person or object.
I cannot find the same hover suggestion as the one you mention. To me the suggestions are: Ils - they (msculine), them, hes; sont - (they) are; nombreux - numerous, large, many.
And I cannot find "de nomberux" in my french-french dictionary either. There is nombreux/nombreuse (de), but not de nombreux.
So, my guess is that it was a faulty hover suggestion! There is no need for the article. At least not in this sentence since it is not a noun but an adjective.