Quantity words, singular vs. plural

[mister.meng]

Hi, I am reading the 4th edition of SC guide, and have learned a lot from the SANAM strategy in S-V agreement chapter. In that session the book also says "treat quantity phrases in the same way as SANAM pronouns", such as "half of the", "50% of the"....etc.

However, I'm wondering how to deal with phrases like "2 pints of milk", "a box of matches", "2 bottles of lemonade". Can the instructors give some explanations about what "quantity phrases" mean exactly?

Thanks