Let $n$ be a positive integer. Find the number of all polynomials $P$ with coefficients from the set $\{0,1,2,3\}$ and for which $P(2)=n$.
1. **Define the sets and polynomials:**
   - Let \( K = \{0,1\}[X] \) be the set of polynomials whose coefficients are in \(\{0,1\}\).
   - Let \( L = \{0,1,2,3\}[X] \) be the set of polynomials whose coefficients are in \(\{0,1,2,3\}\).
   - Let \( P(x) \in L \).

2. **Express \( P(x) \) in terms of polynomials with coefficients in \(\{0,1\}\):**
   - We can write \( P(x) \) as \( P(x) = A_0(x) + A_1(x) + 2A_2(x) + 3A_3(x) \), where \( A_i(x) \in K \) is the polynomial made of the sum of powers of \( x \) whose coefficient is \( i \).

3. **Define new polynomials \( U(x) \) and \( V(x) \):**
   - Let \( U(x) = A_0(x) + A_1(x) + A_3(x) \) and \( V(x) = A_2(x) + A_3(x) \).
   - Thus, \( P(x) = U(x) + 2V(x) \) with \( U, V \in K \).

4. **Consider the function \( f \) from \( K \) to \(\mathbb{N} \cup \{0\}\):**
   - Define \( f \) such that \( f(P) = P(2) \). This function is a bijection because each polynomial in \( K \) evaluated at \( x = 2 \) gives a unique non-negative integer.

5. **Count the number of solutions to \( a + 2b = n \):**
   - We need to count the number of pairs \((a, b)\) where \( a, b \in \mathbb{N} \cup \{0\} \) such that \( a + 2b = n \).
   - For each \( b \), \( a \) must be \( n - 2b \). The value of \( b \) can range from \( 0 \) to \( \left\lfloor \frac{n}{2} \right\rfloor \).

6. **Determine the number of such pairs:**
   - The number of valid pairs \((a, b)\) is \( 1 + \left\lfloor \frac{n}{2} \right\rfloor \), as \( b \) can take any integer value from \( 0 \) to \( \left\lfloor \frac{n}{2} \right\rfloor \).

Conclusion:
\[
\boxed{1 + \left\lfloor \frac{n}{2} \right\rfloor}
\]