For how many integers $n$ between $ 1$ and $2021$ does the infinite nested expression $$\sqrt{n + \sqrt{n +\sqrt{n + \sqrt{...}}}}$$ give a rational number?
1. Let the infinite nested expression be denoted by \( x \). Therefore, we have:
   \[
   x = \sqrt{n + \sqrt{n + \sqrt{n + \sqrt{\ldots}}}}
   \]
   Squaring both sides, we get:
   \[
   x^2 = n + \sqrt{n + \sqrt{n + \sqrt{\ldots}}}
   \]
   Since the expression inside the square root is the same as \( x \), we can write:
   \[
   x^2 = n + x
   \]

2. Rearrange the equation to form a quadratic equation:
   \[
   x^2 - x - n = 0
   \]

3. For \( x \) to be rational, the discriminant of the quadratic equation must be a perfect square. The discriminant \(\Delta\) of the quadratic equation \( ax^2 + bx + c = 0 \) is given by:
   \[
   \Delta = b^2 - 4ac
   \]
   Here, \( a = 1 \), \( b = -1 \), and \( c = -n \). Therefore, the discriminant is:
   \[
   \Delta = (-1)^2 - 4(1)(-n) = 1 + 4n
   \]

4. For \( x \) to be rational, \( 1 + 4n \) must be a perfect square. Let:
   \[
   1 + 4n = m^2
   \]
   for some integer \( m \). Rearranging, we get:
   \[
   4n = m^2 - 1
   \]
   \[
   n = \frac{m^2 - 1}{4}
   \]

5. Since \( n \) must be an integer, \( m^2 - 1 \) must be divisible by 4. Notice that \( m^2 - 1 = (m-1)(m+1) \). For \( m^2 - 1 \) to be divisible by 4, both \( m-1 \) and \( m+1 \) must be even, which implies that \( m \) must be odd.

6. We need to find the range of \( m \) such that \( n \) is between 1 and 2021. We have:
   \[
   1 \leq n \leq 2021
   \]
   Substituting \( n = \frac{m^2 - 1}{4} \), we get:
   \[
   1 \leq \frac{m^2 - 1}{4} \leq 2021
   \]
   Multiplying through by 4, we get:
   \[
   4 \leq m^2 - 1 \leq 8084
   \]
   Adding 1 to all parts, we get:
   \[
   5 \leq m^2 \leq 8085
   \]

7. Taking the square root of all parts, we get:
   \[
   \sqrt{5} \leq m \leq \sqrt{8085}
   \]
   Approximating the square roots, we get:
   \[
   3 \leq m \leq 89
   \]

8. Since \( m \) must be an odd integer, we list the odd integers between 3 and 89 inclusive. These are:
   \[
   3, 5, 7, \ldots, 87, 89
   \]
   This is an arithmetic sequence with the first term \( a = 3 \) and common difference \( d = 2 \). The number of terms \( n \) in this sequence is given by:
   \[
   a_n = a + (n-1)d
   \]
   Setting \( a_n = 89 \), we solve for \( n \):
   \[
   89 = 3 + (n-1) \cdot 2
   \]
   \[
   89 = 3 + 2n - 2
   \]
   \[
   89 = 2n + 1
   \]
   \[
   88 = 2n
   \]
   \[
   n = 44
   \]

Therefore, there are 44 such values of \( n \).

The final answer is \(\boxed{44}\)