Find the largest $K$ satisfying the following:

Given any closed intervals $A_1,\ldots, A_N$ of length $1$ where $N$ is an arbitrary positive integer. If their union is $[0,2021]$, then we can always find $K$ intervals from $A_1,\ldots, A_N$ such that the intersection of any two of them is empty.
1. **Claim**: We claim that \( K = 1011 \).

2. **Upper Bound**: To see that \( K \le 1011 \), consider the intervals \([i, i+1]\) for \(0 \le i \le 2020\). There are 2021 such intervals, and since each interval has length 1, they cover the entire interval \([0, 2021]\). However, any two consecutive intervals \([i, i+1]\) and \([i+1, i+2]\) overlap. Therefore, we can select at most every other interval to ensure no two selected intervals overlap. This gives us at most \(\left\lceil \frac{2021}{2} \right\rceil = 1011\) non-overlapping intervals.

3. **Existence of 1011 Non-Overlapping Intervals**: We now show that for any such \(N\) intervals, we can find 1011 such intervals that are pairwise disjoint.

4. **Key Observation**: For any integer \(i\), there exists an interval \([k, k+1]\) such that \(\lfloor k \rfloor = i\). This is because if there were no such interval, then the elements of \((i, i+1)\) would not be covered, contradicting the fact that the union of the intervals is \([0, 2021]\).

5. **Verification**: Suppose there exists an interval \([a-1, a]\) such that \(\lfloor a \rfloor = i\). If there doesn't exist one, then the elements of \((i, i+1)\) don't get covered. If \(a = i+1\), we are done. If \(a < i+1\), then the interval \((a, i+1)\) doesn't get covered, which is a contradiction.

6. **Selection of Intervals**: For \(i = 0, 2, 4, \ldots, 2020\), choose intervals such that the floor of the lower bound is \(i\). Since the length of each interval is 1, they are disjoint. This gives us 1011 intervals (since there are 1011 even numbers from 0 to 2020 inclusive).

Therefore, we have shown that it is always possible to find 1011 intervals from \(A_1, \ldots, A_N\) such that the intersection of any two of them is empty.

\(\blacksquare\)

The final answer is \( \boxed{ K = 1011 } \)