a mumber of , means "several", is plural : A number of people have left.
by number : She knows all the rooms in the building by number.
in number : We were 30 in number.
number amount/in :We number him amount our friends.
number off : The soldiors number off, straight from the right-hand man.
numbers of : A largy quntity, Numbers of people come to the meeting.
to the number of : There are people to the number of 100 who had taken part in the competition.
without number : I have been there for times without number.
