It is both. You have to learn both of them.

"Candy" is both countable and uncountable. It is uncountable because it is a general food item. However, it is countable if it comes as an individual piece such in a given context .." I will buy a piece of candy. Both are acceptable to say: My mom says," Do not eat too much candy" ( uncountable? or My mom says, " Do not eat too many candies" ( countable )

You can say either, but they have slightly different meanings: "I want a candy" means that you want ONE piece of candy. "I want candy" means that you want ONE or MORE pieces, but in this case, it's not specific as to what candy you mean. "I want candies" means that you want multiple (usually different types) pieces of candy. This is a great example of how confusing the plural (the multiple form of a word) can be in English. Hope this answer helps!

You may say that. You may also say I want some soups if you want more than one kind. You can count types of candy and soups. You can also say "I want candy" and mean many. Candy is not countable.