To expect an appointment with all my child's teachers?

It would be impossible to see every child's parents. It's actually nothing to do with online, it would be the same in person.
I teach 5 year 7 groups this year, so 125 kids in total. Even with a roughly 50% attendance rate that's 60+ appointments
Our school encourages parents to make a 'hit list's who they want to see and book. Prioritise, but also remember you can send an email telling us you couldn't get an appointment as we were fully booked and request a phone call instead. I do this a lot.

Sometimes a teacher will have more than one class in a year group. For us at Key Stage 3 this would mean 64 students but there are 42 slots in an evening (I think). However, our staff would be more than happy to reply to an email with any questions and give feedback to anyone who was unable to get an appointment.

It’s because parents evening counts as directed time and there is a limit to how many hours directed time they can be given in a year, secondary teachers will have a lot of students they teach so to see every parent at every parents evening across every year group may be impossible for some teachers.
the general rule for parents evening is that there shouldn’t be any suprises so if a students parent ‘needs to be spoken to’ this is usually done when it comes up rather than kept for the parents evening.

But often it’s the patents who you need to talk to who don’t bother with parents evening.

I think the teachers shod be responsible for who gets a slot. They know who's parents need speaking to

That's awful it shouldn't be like getting tickets to see Taylor swift!

In the olden days when kids walked round with a peice of paper and organised it themselves you were much more likely to get a slot as half the kids couldn't be bothered.

What is the time range given for the appointments? Are there any teachers you specifically wanted to see? I'd focus on trying to get to see two or three teachers of subjects you have concerns about.

You do need to be there bang on the dot to get a slot at my kids secondary. I always set a reminder in my calendar for the morning when booking’s open and then I set an alarm for 5 mins before. I guess yes they should have enough slots for 1 per child but at our school you get a written update via email if you can’t get a slot.

edited to add we get an email to say booking will open at 6pm on 29th Feb for example around 2wks before. So then you can be ready to grab the time slots you want