"One hundred of them are very well."
Translation:Cento di loro stanno benissimo.
Essere is for more permanent things, and Stare is things that are subject to change. I try to remember how to use them by "Io sono Audra," (Essere)-"I am Audra" because I am always going to be Audra, and "Sto bene" (Stare)- "I am well" because I am probably not ALWAYS going to be well, and it is subject to change. I hope this helps.
I can't say for sure as I am not a native speaker. They both mean "very well" but I gather that "benissimo" is used to describe how people feel (in a positive sense) whereas molto bene is just the standard adverbial phrase for "very well" used in other situations. Because it uses the intensifier suffix -issimo, "benissimo" also can means things like "great", "excellent" etc. when describing how you feel, so it has that sense of exaggerated well-being that we also try to convey in English when describing how we feel.
However if you use "molto bene" when describing how you feel it is quite likely it will be in a negative sentence e.g. non mi sento molto bene if the following link to Reverso Context is any guide:
In Italian "cento" and "mille" are cardinal numbers. It's never "un cento/mille" just as it's never "un dieci/venti/cinquanta". On the other hand "milione/bilione/miliardo" are nouns and are used as "un milione/bilione/miliardo" just as in English "a/one hundred/thousand/million"
Cento di loro sono molto bene. This is Googles tranlation Google is not always correct. However It tells me that benissomo at the end of the scentence means ( well benidicted) which I belive to to inaccurate. Google is useful at times.I don't know why a big company like them cannot be 100 % correct all the time