The ROWS function is fully automatic. When you provide a range to ROWS, it will return a count of all rows in the range. In the example, the formula in F5 returns 6, because there are 6 rows in the range B5:C10:
=ROWS(B5:C10) // count rows
ROWS counts the number of rows in any supplied range and returns a number as a result. For example, if we provide all of column A in a range, Excel returns 1,048,576 the total number of rows in an Excel worksheet.
=ROWS(A:A) // returns 1048576
To count columns in a range, see the COLUMNS function.