Whereas COUNT counts only the cells that have numbers in them, COUNTA and COUNTBLANK are a little more loose with the definition of what they count. They count all cells and report on the information.
COUNTA counts cells that are not blank.
You might use blank spaces to create a view that separates certain types of data so that it’s easier on the eye, such as the T-shirt example below.
However, if you want to know the total number of sizes and colors, you don’t want to count those spaces.
In that case, you would use =COUNTA(B2:B18), which would give you a number of 15
COUNTBLANK does exactly what you would expect it to do: it counts the number of blank cells.
=COUNTBLANK(A1:C18) gives me 6 because I have 6 blank cells.