Learn French: Le chiffre d'affaires augmente de plus en plus.

It literally translates to "figure of businesses" or "number of deals". In French business vocabulary, les affaires refers to business, sales, or deals in general. Because a company conducts multiple transactions, affaires is plural here. This fixed expression is the standard way to say "turnover" or "revenue" in French.

Do I pronounce the 's' in both instances of plus in de plus en plus?

Yes, but they sound different! You must make a liaison between the first plus and en, so it sounds like a 'z' (de plu-z-en). The final 's' in the second plus is also pronounced, but as a soft 's' sound (plusss), which is standard for the end of the fixed phrase de plus en plus.

Could I use grandit (grows) instead of augmente here?

You shouldn't. In French, grandir is typically reserved for physical growth, like a child, an animal, or a tree getting taller. When talking about values, prices, numbers, or a chiffre d'affaires going up, you should always use augmenter.