The correct answer is "There are two bottles of milk." The sentence is about the number of bottles, not about the amount of milk. The words "of milk" acts as an adjective to describe what kind of bottles, but the phrase is about bottles.

By itself, milk is usually uncountable (so singular), but we can use counters such as bottles, glasses, grams, ounces, kinds, etc., which are countable and can be plural. Sometimes “milk” can be pluralized, in which case a counter is implied. This is generally true of uncountable nouns in English (and other languages with such distinctions). So, in answer to your question, "there are two bottles of milk."

哦哦，这样，谢谢指点，有机会多交流哈~

嗯，我明白你想问的是，“milk不可数，而bottle可数，那这里到底该用is 还是are”。下面那个黄头发的帅哥解释得很详细了。 不过我说点题外话，口语中，好多人不管后面接的是什么，直接there's 了，你可以理解为他们把there is， there are, there has 统统缩成了there's。所以经常会听到 There's a lot of people. There’s two hundred tickets left. There's three ducks on the pond. There's lots of reasons to love him 这虽然不合规，但是母语使用者经常这么用，我估计再过几年，英语词典可能会增加相应的解释。

Answer 2: Actually, I’m going to expand on my answer, since it’s not really as clear-cut as some people might at first suppose. In your example, it was clearly plural, but in real life, it could often go either way: A: How much milk did you drink? B: About four liters. A: Four liters? Four liters is a lot of milk! In this case, we would usually use a notional singular, since we’re talking about the milk, not the “liters”. In your first example, there are two actual, physical bottles, but in my example, liters are just a measurement for the quantity of milk. The liters don’t necessarily exist — they’re just an abstract concept to quantify the milk. Perhaps it was a single one-gallon bottle of milk that B drank :) It’s the same idea with money. We say: “Ten trillion dollars is a lot of money.” Note that there are not necessarily any actual, physical dollar bills — it’s just a figure on an account statement somewhere.