This page gives a formula $$B = \frac{\mu_0 I}{2R}$$
for the field at the centre of a current carrying loop in vacuum. Earth's magnetic field is about $10^{-4}T$, the radius of Mars is $3.4\times 10^6 m$ and $\mu_0$ is $4\pi\times 10^{-7}$. Solving for $I$ we get about $5\times 10^8$ -- 500 million amps.
However, Mars is not a vacuum, and its core is iron. Depending on its purity and temperature (which are not, I think, very well known) that could increase the field by up to 1000-fold or more, so a relatively modest current of a few hundred thousand amps might suffice, or even no current once the core was magnetised.
All of which said, I am not convinced that a magnetic field is a major short-term consideration in terraforming Mars. In the very long term it might reduce loss of light gasses from the top of the atmosphere and steer away some kinds of radiation. It doesn't help with UV though, and a thicker atmosphere would deal with charged particles anyway.