that’s currently not supported (or at least, not very easy) with the plot_topo method. It has a legend parameter, which will add an empty set of axes that have the axis ticks (all the non-empty axes have the same x & y limits), and no way to add individual subplot titles. It was designed for plotting tens or even a hundred sensors at once, when adding all those tick marks and titles would usually be bad / cluttered. In fact, it doesn’t even create separate subplots for each sensor — this effect is simulated to make drawing more efficient.
For your example where there are only 7 axes, I can see why it might be desirable to have them. Try mne.viz.plot_compare_evokeds with parameter axes='topo' and see how close that gets you. If you’re still unsatisfied, post back here and we’ll try to help some more.