According to the definition of the Jacobian on Wikipedia:
I've seen it written elsewhere that the members of a Jacobian matrix are all real numbers. But in case $f: \mathbb{R}^n \rightarrow \mathbb{R}^m$ for some $m > 1$, then aren't each of the $\partial f_i / \partial x_j \in \mathbb{R}^m$?
Question:

No, because each $f_i$ is a real function, and therefore each $\frac{\partial f_i}{\partial x_j}$ is a real number.