J explained the grammar difference. As far as use goes, you would say "so much fruit" when you want to say that there is a lot of fruit in one place, a large amount, like too much to eat. If you saw fifty apples and a hundred pears and twenty bunches of bananas, you would use that. If you said "so many fruits" that sounds more like you are saying that there are many different kinds of fruit: apples, oranges, pears, kiwis, bananas, pears, pineapples, grapes, melons, figs, berries, watermelons, mangoes, papayas, apricots, pears...

Fruit(singular) is a non-countable noun Fruits(plural) is a countable noun A daily fruit is healthy for you. Fruits and vegetables are nutritious.