=SUMPRODUCT(--ISERR(rng))

If you need to count the number of cells that contain errors, you can use the ISERR function, wrapped in the SUMPRODUCT function. In the generic form of the formula (above) rng represents the range of cells in which you'd like to count errors.

In the example, the active cell contains this formula:

=SUMPRODUCT(--ISERR(B4:B8))

SUMPRODUCT accepts one or more arrays and calculates the sum of products of corresponding numbers. If only one array is supplied, it just sums the items in the array.

The ISERR function is evaluated for each cell in rng. The result is an array of TRUE / FALSE values:

{TRUE;FALSE;TRUE;FALSE;FALSE}

The -- operator (called a double unary) coerces the TRUE/FALSE values to zeros and 1's. The resulting array looks like this:

{1;0;1;0;0}

SUMPRODUCT then sums the items in this array and returns the total, which in the example is the number 2.

Note: ISERR counts all errors except #N/A. If you want to also count #N/A, use the ISERROR function instead of ISERR.

You can also use the SUM function to count errors. The structure of the formula is the same, but it must be entered as an array formula (press Control + Shift + Enter instead of just Enter). Once entered the formula will look like this: