Solution
= Solution
A <primitive root> modulo $p$ is a generator of the cyclic multiplicative <group> $(\mathbb Z/p\mathbb Z)^\times$, so it has order $p-1$. If $g$ is one generator, the generators are exactly $g^k$ with $\gcd(k,p-1)=1$. Hence there are $\boxed{\phi(p-1)}$. For $p=2$ the <group> is trivial and its unique element is its generator, consistently with $\phi(1)=1$.