What is the least number that must be added to $1500$ to turn it into a perfect square?

Let's look at the squares of integers near $\sqrt{1500}$:
$$38^2=1444$$

$$39^2=1521$$

Since $1500$ lies between $1444$ and $1521$, the next immediate perfect square is $1521$.
The minimum amount to add is the difference:

$$\text{Number to add}=1521-1500=21$$