so i got my paper one today - but only one? Don't I need one for each family member?
The norm is one form (or online return) for the whole household - you don't need a form per person.
How are we meant to be filling it out now?
The judicial review win means that people must answer question 3, about their sex, according to their birth certificate (or gender recognition certificate).
The letter to send with the paper copy sexinthecensus.org/on-census-night/ is about registering a protest that it should be your actual sex, regardless of GRC. The reason for making it a letter is that comments written on the form will simply be ignored; forms will be computer-read and provided your answer can be worked out from the content of the boxes anything extraneous will be ignored.
It's not like an election where a human sees every form and any spoiled ballot goes to the returning officer.
So there's no point writing comments on the form; you need to send the covering letter, because it requires a response.
If you ask for a paper copy and it doesn't arrive in time, just wait - it will - and if it doesn't a census officer will almost certainly visit to see why your household hasn't completed the form, and will be able to give you a paper copy on the spot.
It's not in any sense an offence to fill the form out late - you might (in normal times anyway) be away on holiday on the day and provided you fill it out as soon as you can thereafter that's fine. So saying to the census officer "I requested a paper form but it hasn't come yet" is absolutely fine.