Finally got my registration through. In the original form it asked how many birds you have, since then I've lost one (old age, they're all close to nine years old now), but the registration doesn't mention numbers nor does it ask you to tell them when you lose any. I presume you would if it's notifiable like bird flu.
Do you need to tell them when the size of your flock changes? I've only got two left anyway now.
According to this (and the notice they send you)
https://www.gov.uk/guidance/register-as-a-keeper-of-less-than-50-poultry-or-other-captive-birds#after-you-have-registeredAfter you have registeredIf your details have changed, you’ll need to tell APHA within one month when:
you no longer keep or own birds
your contact information changes
you change the species of poultry or other captive birds you keep
you change the purpose or purposes for which you keep birds
you start keeping 50 or more poultry or other captive birds
So, I think probably not myself.