DD's bat mitzvah was due to happen June this year, we've been able to move to end of next June as synagogue was starting to allow girls to have their at 13 (for parity with boys, rather than 12 as previously) from next year anyway.
DH still seems to think we're going to be able to hav 100+ guests by then, and while I reckon things will have improved, I think we'll be lucky if we can even get to 50. They did start have small, socially distanced BMs at synagogue allowing 20 guests (which just about covers our close family, thankfully) when national rules allow, so that should be our bare minimum, but I'm not convinced they'll permit a great deal more by next summer, even with a vaccine as distancing will still be there and the room isn't getting any bigger. We'll basically want to send invites late March and I'm not sure how clear things will be by then. Probably not very.
The problem is the bulk of planned guests are DH's mates, and they're all quite interlinked, so as soon as you get past his best mate, then suddenly it's 100+ people once you include everybody's kids because you can't invite A, and not invite B, C and D etc etc and I think he's going to have to think less in terms of everyone he could possibly invite, and more about who he can invite as a core if permitted - people will be understanding, I think, that we've had to restrict numbers.