Here's a start.

10,000 people with 2 cups each gives total cups being 20,000.

Total volume of pink lemonade required is 7,080,000 ml

Now only 15ml per cup is to be cordial i.e 15/254 = 4%, that means 96% of the cup is lemonade.

So what is 4% and 96% of 7,080,000? How many bottles is then required?