=SUMPRODUCT(--NOT(ISERR(rng)))

If you need to count the number of cells that contain errors, you can use the ISERR and NOT functions, wrapped in the SUMPRODUCT function. In the generic form of the formula (above) rng represents the range in which you'd like to count cells with no errors.

In the example, the active cell contains this formula:

=SUMPRODUCT(--NOT(ISERR(B4:B8)))

Here's how the formula works:

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. Without the NOT function, the result is an array values equal to TRUE or FALSE:

{TRUE;FALSE;TRUE;FALSE;FALSE}

With the NOT function, the result is:

{FALSE;TRUE;FALSE;TRUE;TRUE}

This corresponds cells that do not contain errors in the rng.

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

{0;1;0;1;1}

SUMPRODUCT then sums the items in this array and returns the total, which in the example is the number 3.

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:

Don't enter the braces {}, they are entered for you when you press Control + Shift + Enter.