What is the classification of the words such as "group of", "bundle of", "number of" and also the right form of verb after using this words?

First example:

1. A bundle of roses is the most beautiful thing one can have in the morning.

2. A bundle of roses are the most beautiful thing ...

Second example:

1. The number of books are four.

2. The number of books is four.

Which of the above sentences are correct?

Solution 1:

"Bundle" and "(the) number" are collectives, or collective nouns.

A bundle of roses is the most beautiful thing one can have

is correct, as "a bundle" is a singular thing, even if composed of many parts. "Bundles" would be plural.

The number of books is four.

"The number" will always be singular, as it refers to a specific quantity. The article "the" limits "number" to one specific thing.

"A number" will always be plural, as the reference is to an indefinite quantity. "The number" is a collective, however "a number" is better classed a quantifier.