Question: How to count unique values in a list?

Introduction:
Column A (A1:A15) are random numbers between 0 and 10.

Answer:Cell B18 is {=SUMPRODUCT(1*(COUNTIF(A1:A15,A1:A15)=1))}
(Press CTRL+SHIFT+RETURN after the formula is typed in the cell)

COUNTIF(A1:A15,A1:A15) checks if a value is only represented once in the list (A1:A15). The result is either TRUE (1) or FALSE (0)

The sumproduct sums the true results.

SUMPRODUCT(array1;array2;...)
Returns the sum of the products of the corresponding ranges or arrays

COUNTIF(range;criteria)
Counts the number of cells within a range that meet the given condition