Yes, patches to different files can qualify as a patch series. Loosely defined, a patch series is a series of patches which should be applied one after the other; more precise definitions depend on the target project’s practices.
One common pattern for patch series is to add a new feature, possibly as several patches, then use it; the “define” versus “use” parts will often touch different files.
In most cases, I would use a patch _series_ to submit multiple patches which don’t make sense without each other. If individual patches are useful on their own, I would submit them individually. The general use for a patch series is to split up a complex change into multiple steps, in order to simplify review (whether the initial review, or “review“ when people look at the history).