How does a reader know how many tags are there in its range in RFID

How will reader know the no of tags trying to access it,and can anyone please explain me the mechanism

the reader sends out multiple blind querries, and all of the tags in the vicinity respond with their serial number. the reader then tells you how many serial numbers it found.

But what happens if more than one tags reply to reader in same time ,maybe there will be a collision and how does reader know that there is a collision

The procedure is a bit more complex. You should look into an RFID book.
But the basic idea is:
The reader tell all tags to choose a random number in a given range.
Then the reader asks all tags if some tag has choosen the random number 1. If a has choosen this number it replies its serial number.
The reader goes through the whole range. If two or more tags have choosen the same number they will reply at the same time and collision occurs. Then the reader has to start again.
If all numbers of the range are questioned the procedure is finished and the reader knows all tags in the field.