It can be accompanied by classifier "cái". The answer that contains "cái" is already accepted but generally, classifier is optional.
It's not about grammar rule but it's about the preference. Vietnamese speakers tend to ignore classifier cái when the objects (glass, bottle...) are... specific enough. For instance: tôi cần một cái chai (I need a bottle - so you need classifier), however: tôi cần một chai rượu Vodka (I need a bottle of Vodka - no classifier here). On the other hand, it's not wrong to include "cái".
What does this mean? Like you're ordering, or you're stealing the glass of juice??? Because if it's supposed to be ordering in a restaurant, the correct English would be "We WILL take a glass of juice." (We'll). But even that is awkward, as it is weird for more than one person to share one glass of juice, so the meaning here is really unclear. In English, the meaning would be that two or more people are stealing a glass of juice. Is this the intended meaning of this sentence?
Im not far along in my studies but i see two possibilities.
This is just practise to learn more combinations of words and how they go together. We've learned a lot of thungs that we may never say but they help us learn.
This is how they order juice at a restaurant when there is another person or more with them. It doesnt mean its a weird way to order. You are looking at a direct translation which will make a lot of phrases in a lot of languages look weird. While it might mean we steal a glass of juice to english speakers, it could just be ordering to them