Given that $\log_{10} \sin x + \log_{10} \cos x = -1$ and that $\log_{10} (\sin x + \cos x) = \textstyle \frac{1}{2} (\log_{10} n - 1)$, find $n$.
1. Given the equation:
   \[
   \log_{10} \sin x + \log_{10} \cos x = -1
   \]
   Using the properties of logarithms, we can combine the logs:
   \[
   \log_{10} (\sin x \cos x) = -1
   \]
   This implies:
   \[
   \sin x \cos x = 10^{-1} = \frac{1}{10}
   \]

2. Using the double-angle identity for sine, we know:
   \[
   \sin(2x) = 2 \sin x \cos x
   \]
   Substituting the value of \(\sin x \cos x\):
   \[
   \sin(2x) = 2 \left(\frac{1}{10}\right) = \frac{1}{5}
   \]

3. Next, we need to find \(\sin x + \cos x\). We use the identity:
   \[
   (\sin x + \cos x)^2 = \sin^2 x + \cos^2 x + 2 \sin x \cos x
   \]
   Since \(\sin^2 x + \cos^2 x = 1\), we have:
   \[
   (\sin x + \cos x)^2 = 1 + 2 \left(\frac{1}{10}\right) = 1 + \frac{1}{5} = \frac{6}{5}
   \]
   Therefore:
   \[
   \sin x + \cos x = \sqrt{\frac{6}{5}}
   \]

4. Given the second equation:
   \[
   \log_{10} (\sin x + \cos x) = \frac{1}{2} (\log_{10} n - 1)
   \]
   Substitute \(\sin x + \cos x = \sqrt{\frac{6}{5}}\):
   \[
   \log_{10} \left(\sqrt{\frac{6}{5}}\right) = \frac{1}{2} (\log_{10} n - 1)
   \]

5. Simplify the left-hand side:
   \[
   \log_{10} \left(\sqrt{\frac{6}{5}}\right) = \frac{1}{2} \log_{10} \left(\frac{6}{5}\right)
   \]
   Therefore, the equation becomes:
   \[
   \frac{1}{2} \log_{10} \left(\frac{6}{5}\right) = \frac{1}{2} (\log_{10} n - 1)
   \]

6. Multiply both sides by 2 to clear the fraction:
   \[
   \log_{10} \left(\frac{6}{5}\right) = \log_{10} n - 1
   \]

7. Add 1 to both sides:
   \[
   \log_{10} \left(\frac{6}{5}\right) + 1 = \log_{10} n
   \]

8. Rewrite 1 as \(\log_{10} 10\):
   \[
   \log_{10} \left(\frac{6}{5}\right) + \log_{10} 10 = \log_{10} n
   \]

9. Use the property of logarithms to combine the logs:
   \[
   \log_{10} \left(\frac{6}{5} \times 10\right) = \log_{10} n
   \]

10. Simplify the argument of the logarithm:
    \[
    \log_{10} \left(\frac{6}{5} \times 10\right) = \log_{10} 12
    \]

11. Therefore:
    \[
    \log_{10} 12 = \log_{10} n
    \]

12. Conclude that:
    \[
    n = 12
    \]

The final answer is \(\boxed{12}\)