There is no built-in function for counting the total numbers of cells in a range, so you need to use the ROWS and COLUMNS functions together. In the example, ROWS returns the total number of rows in B5:C10 (6), and COLUMNS returns the total number of columns in B5:C10 (2). The formula multiplies these values together and returns the result:
=ROWS(B5:C10)*COLUMNS(B5:C10) // returns 12
The ROWS and COLUMNS functions can work with any sized rectangular range. For example, the formula below uses the ROWS function to count all rows in the column A, and the COLUMNS function to count all columns in row 1:
=ROWS(A:A)*COLUMNS(1:1) // count all cells in worksheet
ROWS returns a count of 1,048,576 and COLUMNS returns a count of 16,384, to the final result is 17,179,869,184.