O.K It's a bit difficult for me to grasp because for instance you would normally say there is a load of bricks. You wouldn't say that a load of quantifies bricks and therefore say there are a load of bricks. Still I s'pose that's the English laguage for you.
O.K I wonder if I can explain it better.
When we speak of crowds,we say there is a crowd of people or there are crowds of people. That's because the word crowd (even though it quantifies many people) is singular, and so we say there is a crowd. The word crowds on the other hand, is plural, so we say there are crowds.
What we are really saying is that there is a crowd and it has people in it,or there are crowds and they have people or peoples in them. So why all of a suden when what we are really saying is : There is a number and that number is made up of people does it become O.K. to say of a number,singular, that it are? Or there are a number. I could understand it more easily if they said there are numbers
It's the same with, there is a box of tools and there are boxes of tools. There is a herd of horses and there are herds of horses and so on. A herd,a box,a crowd, a lorry load, a bucket of, a cubic metre of. All quantify a number of things but are preceeded by is, because they are singular.
I'm obviously struggling with this one so thanks for your efforts.
"number" is different. "a number of things" expresses a plural meaning. Consider,
EX: There is a crowd of people outside.
EX: There is a crowd outside.
"of people" isn't all that necessary, which means "crowd" is the subject, and the reason it agrees in number with the verb. Now consider our example,
EX: There are a number of things wrong.
EX: *There are a number wrong.
"of things" is required. Omit it and the resulting sentence is ungrammatical, which tells us that "a number" isn't the subject. "things" is the subject. Omit the modifier, though, and the sentence is grammatical:
EX: There are things wrong.
Cf. There is a crowd outside.
There are two kinds of "of" structures:
 crowds of people (head + modification)
 number of people (modification + head)
ctually it helped a lot. Although I didn't agree with,or pehaps was unable to understand, all you said, It did make me think hard and encourage me to discuss the subject with others and draw conclusions. So thanks for your patience.