since you have a hollow metal doughnut, i think you need to treat it as one loop
then, as in the hyperphysics article, B = mu N I /2 pi R
where N i2 one
as for the units, I in amperes, R in meters, mu is 4 pi x 10^-7 Tm/A (tesla meter per amp)
then B is in tesla
i suggest there is only one turn because there is no wire wrapped around a form, as in the usual torroid
and because a look at the topology:
starting from a hollow metal doughnut, cut across the doughnut and straighten it, to get a cylinder
now cut along the length of the cylinder and one has a sheet
current enters the sheet along one long side and exits along the other long side
the current will spread out from the wire bringing the current in (like cars entering a multi-lane highway from a single lane)
after a very short distance, the current density/length will be constant
likewise, at exit, all of the current will "squeeze" down into the outgoing wire
hence, we can treat your torroid as a one turn device
mathematically, if we assume N turns, the current per turn will be (incoming current)/N
so in the equation, N is N and I is (incoming current/N)
N cancels out