Find the number of squares in the sequence given by $ a_0\equal{}91$ and $ a_{n\plus{}1}\equal{}10a_n\plus{}(\minus{}1)^n$ for $ n \ge 0.$
To find the number of squares in the sequence given by \( a_0 = 91 \) and \( a_{n+1} = 10a_n + (-1)^n \) for \( n \ge 0 \), we will analyze the sequence modulo 8 and modulo 1000.

1. **Base Cases:**
   - \( a_0 = 91 \)
   - \( a_1 = 10a_0 + (-1)^0 = 10 \cdot 91 + 1 = 911 \)
   - \( a_2 = 10a_1 + (-1)^1 = 10 \cdot 911 - 1 = 9109 \)

2. **Induction Hypothesis:**
   We will prove by induction that for \( k \ge 2 \):
   - \( a_{2k} \equiv 909 \pmod{1000} \)
   - \( a_{2k-1} \equiv 091 \pmod{1000} \)

3. **Induction Step:**
   - Assume \( a_{2k} \equiv 909 \pmod{1000} \) and \( a_{2k-1} \equiv 091 \pmod{1000} \).
   - For \( a_{2k+1} \):
     \[
     a_{2k+1} = 10a_{2k} + 1 \equiv 10 \cdot 909 + 1 \equiv 9090 + 1 \equiv 9091 \pmod{1000} \equiv 091 \pmod{1000}
     \]
   - For \( a_{2k+2} \):
     \[
     a_{2k+2} = 10a_{2k+1} - 1 \equiv 10 \cdot 091 - 1 \equiv 910 - 1 \equiv 909 \pmod{1000}
     \]

4. **Quadratic Residue Analysis:**
   - For \( a_{2k} \equiv 909 \pmod{1000} \):
     \[
     909 \equiv 5 \pmod{8}
     \]
     Since 5 is not a quadratic residue modulo 8, \( a_{2k} \) cannot be a perfect square.
   - For \( a_{2k-1} \equiv 091 \pmod{1000} \):
     \[
     091 \equiv 3 \pmod{8}
     \]
     Since 3 is not a quadratic residue modulo 8, \( a_{2k-1} \) cannot be a perfect square.

5. **Conclusion:**
   - For \( n \ge 3 \), \( a_n \) is not a square.
   - Checking the initial terms:
     - \( a_0 = 91 \) is not a perfect square.
     - \( a_1 = 911 \) is not a perfect square.
     - \( a_2 = 9109 \) is not a perfect square.

Thus, there are no perfect squares in the sequence.

The final answer is \(\boxed{0}\).