what do I tag this as and why
Sure seems like rape to me, because no means no.
I know there’s probably some ABO folks who will see this as “if you’re in heat, that’s an open invitation and it’s not rape when someone rapes you against your will even if you’re begging them to stop”, but … woof. She already said she didn’t want him, so how much ‘no’ do you need?
If the final caption from her were something more like “sure, if you think that will help” then sure, not rape. But as it is, no means no.
how many pages of a comic like that would be tagged with something related to this
Just those pages on which there is nonconsensual sex. If the next page she is begging him for more, that page is not rape.
We don’t tag based on themes in a sequential comic - like, we don’t tag an image “lesbian” because two of the female characters in the comic eventually have sex but aren’t even in this particular page - if there’s no rape on some other page, then there’s no rape on that page.
Maybe there might be ‘after rape’, like there’s ‘after care’, but that seems like more of a image-by-image question.
And while there are probably people who would enjoy other parts of that comic who have rape filtered and thus would miss this page, if they find they like the comic but some of the pages are spoilered then it’s their call if they like the comic enough that they want to see pages that include non-consensual sex.