What takes longer? Cooking one or eight turkey steaks?

It's going to depend on size. How large is the 1 turkey steak? How small are the 8?

If they are all approximately the same size, then the 8 will take longer due to the diffusion of heat. You've got 8 equal pieces that have to heat up as opposed to just 1.

If the 1 piece was much larger and thicker than the 8, you might be looking at possible a longer cook time for that one piece. It's difficult to estimate since the turkey steaks are not uniformly shaped and all ovens behavior differently.