Suppose $C$ is a smooth compact complex curve, and let $J$ be its Jacobian. Then there is a theta divisor $\Theta$ in $J$. Such divisor depend on a point $p\in C$.

Question. How to calculate the dimension of the set of divisors on $J$ linearly equivalent to $\Theta$? In other words, what is $dim( H^0(J,\cal O(\Theta)))$?