Answer
$1.4\times10^{12}$ protons.
Work Step by Step
Find the period T, the time for a charge to complete one revolution.
$$T=\frac{d}{c}=\frac{6300m}{3.00\times10^8m/s}=2.1\times10^{-5}s$$
In a time T, the charge Q = Ne passes a point in the ring once, and this comprises a current of 11 mA.
$$0.011A=\frac{N(1.61\times10^{-19}C)}{ 2.1\times10^{-5}s }$$
$$N=1.4\times10^{12}\;protons $$