How much bonding can a ceiling take?

Alot easier way is just to use plasterboard these days

I would recommend pulling down whole ceiling and reboarding as it is already failing so to repair that area it would only be a matter of time before more cracks appeared. If you do decide on a short term fix then I would try to fix 9mm plasterboard (directly on to the laths but ensuring screwed into joists) to make up the depths with a thinner coat of bonding over that to level off with existing ceiling, scrim the joins and reskim. A mechanical fix on the boards to the joists to ensure stability, less mess, quicker and cheaper.

Some times it is easier to overboard the original ceiling, it turns out neeter and you will cut out the hard work of bonding and adding weight to the ceiling, it would only need skimming and look professional.

Overboard the whole ceiling and then skim, it'll turn out a better job.

It really depends on how big the area is, if it is that thick and the area is huge it could take bags and bags of it. Lath and plaster is tricky. I find that pulling down an reboarding is the easiest but expensive. Over boarding also an option but the ceiling could already be all over the place hence the taking down and reboarding.

No, no, no, no,no. Don't chance it for one minute. Overboard with 12.5mm boards. Standard practice for spreads is that you don't put 9mm boards on a ceiling. Too thin and prone to sag. Overboard with 12.5mm board or timber frame a false ceiling underneath to board onto. The cheapest option doesn't mean it's the right one.