That depends on the tense of your sentence could be any form of a verb. I saw a flock of sheep. I have seen a flock of sheep. I have a flock of sheep. With singular or plural verb there is often a...
The simple basic rule is :
in the singular - add 's (apostrophe s) eg the boy's ball or the ball of the boy
in the plural - add ' (apostrophe only) eg the boys' ball or the ball of the boys.
There...
A collective noun is used to refer to a group of people or things. The opposite of a collective noun is a countable noun. If I say there is a flock of birds, you really don't know how many I mean...