You normally have one patch per commit. The patch is the diff between that commit and its parent. (I forget how git format-patch handles the case where there are two parents.)
As per [0] merge commits are dropped:
Note that format-patch will omit merge commits from the output, even if they are part of the requested range. A simple "patch" does not include enough information for the receiving end to reproduce the same merge commit.
I originally thought it would use --first-parent (so just diff vs the first parent, which is what I would want) but apparently no! It is possible to get this behaviour using git log as detailed in this great write-up [1].
[0] https://git-scm.com/docs/git-format-patch#_caveats
[1] https://stackoverflow.com/questions/2285699/git-how-to-creat...
I'm thinking in terms of what I often see from people I work with, where a PR is normally made up of lots of small commits.
I think the point I always get stuck on is how small is "small" when we're talking about commits/patches. Like if you're adding a new feature (to anything, not necessarily the Linux Kernel), should the entire feature be a single commit or several smaller commits? I go back and forth on this all the time, and if you research you're gonna see a ton of different opinions. I've seen some people argue a commit should basically only be a couple lines of code changed, and others argue it should be the entire feature.
You commonly hear Linus talk about commits/patches having very detailed descriptions attached to them. I have trouble believing people would have time for that if each commit was only a few lines, and larger features were spread out over hundreds of commits.
Often, a change to a new working state is necessarily bigger than a couple of lines, or one of the lines has to get removed later.
I don't want to have to say, "Hmm, I wonder if this will work at the end of the file?" and spend a long time figuring out that it won't, then see that the problem is fixed later in the patch series.
Other people may have other preferences.