I don’t think semver is particularly relevant: workflows don’t usually have a public API.
Sure. You can increment it on every release if you want. Google Chrome’s on version 64 already.
As a rule, new major versions tend to be synonymous with paid upgrades, which isn’t relevant to free software.
If your workflow is associated with an app, you might use the same major version as the app for workflow versions that work with that app version.
Otherwise, I don’t think it matters much as long as the version numbers always go up…
I tend to go with “significant rewrite/new features = new major version”, but only because that’s what I know from most apps. I haven’t actually tried to develop a concrete concept for versioning.