Count is aggregator funciton to calculate number of rows and can be nested with one other aggregate function.
Syntax

COUNT( port [, filter_condition] )
or
COUNT( * [, filter_condition] )

Argument	Required/Optional	Description
port	Required	Any datatype except Binary. Passes the values you want to count. You can enter any valid transformation expression.
*	Optional	Use to count all rows in a transformation.
filter_condition	Optional	Limits the rows in the search. The filter condition must be a numeric value or valuate to TRUE, FALSE, or NULL. You can enter any valid transformation
expression.

Note :
If you apply the asterisk argument, this function counts all rows, regardless if a column in a row contains a null value. (Simliar to COUNT(*) in databases )

Group By
COUNT groups values based on group by ports you define in the transformation, returning one result for each group. If there is no group by port COUNT treats all rows as one group, returning one value.

Example 1 : Find number of students who scored more than 90 marks and NULL denotes they did not appear for the exam
Student name can be null (Special case)

COUNT(student_name,marks > 90)

Student_Name	Marks
Edwards	90
Shawn	92
Edwin	94
Murray	NULL
Eliza	89
NULL	99

Output : 2

Example 2 : In the above example NULL is not considered, to consider NULL's

COUNT(*,marks > 90)

Student_Name	Marks
Edwards	90
Shawn	92
Edwin	94
Murray	NULL
Eliza	89
NULL	99

Output : 3

Example 3 : For the above data, find the number of students present in the class

COUNT(*)