"Many argue that the university's only judge the seriousness of the strike by how many people register as having been on strike. But I don't know whether this is actually true."
In my place, it appears to be true. Strike data per department are collated centrally and circulated to all HoDs and above. Data from previous strikes were collated in advance of this one and emailed around to suggest that there would not be much support, accompanied by an email stating a very hard line on ASOS. The pro-VC also personally takes the view that no strike declaration = no support for the strike, even where disruption is still being caused by that person (e.g. they are not showing up for teaching).
I think this is partly a matter of limiting the financial impact - I can't speak for my own department as I have nothing like the seniority required to have access to strike forms, but in DH's very few people have declared for every day. Most have declared for teaching days only. These people definitely support the strike, but the pro-VC would count them as not supportive.
The logic of industrial action in universities is very different from the classic narrative of unionism in factories. In a factory, a lack of labour shuts the place down, hurting the employer's profits because nothing is manufactured. In a university, the employer actually benefits from the strike, since they get to keep money they would otherwise have spent on wages without losing any income (tuition fees having already been paid). This seems to me to create an important difference between the two types of action.
Additionally, in a university one does not sell a temporal quantum of labour in the same way as in a factory. In spite of the best efforts of Stern, some of the capital from the work remains personal to the individual, and there is arguably self-harm as well as institutional harm if this is not completed. There is also an amount of care work that goes on that is not as easy to avoid, since a student who is in need of crisis support cannot simply be told to wait until the strike is over.
Some could argue that, by taking a course of action that maximises disruption (striking on teaching days) while minimising pay impacts (working from home, paid on other days), they are actually inflicting maximum damage on the institution, and are able to sustain industrial action for a longer time than would otherwise be possible. This is not what I personally have done, but I can definitely see an argument for it being a sensible course of action.
Finally, the one thing the universities have run scared of in this particular fight seems to have been reputational/PR damage - should the tide of public opinion, and especially student opinion, turn against the dispute, I suspect that attitudes might change very rapidly.
I'm quite scared at the prospect that we could actually lose this deal and emerge from this with nothing.