Big emulsion job

Depends on the paint chosen. Persumably it will be a white matt but trade and diy paints need different amounts. It gives details on the can.

Probably not. For most colours you need white base coat and two coats of paint.

It usually says on the container, something like 10% water. If the plaster has a very polished finish, increase the water content. Make sure plaster is completely dry first, say until it returns to the light colour plus a week in the case of a skim (longer for scratch coat too).

Possibly -- I have got away doing this with brilliant white on ceilings. Not all colours have the same covering power.

It'll tell you on the tin.

Depends on the paint and the colour. I always get B&Q cheapo for the watered down first coat and then quality (i.e. expensive) paint for the main coat. Unless I'm feeling particularly bored or it's a very strong colour, all that is necessary is a small roller in the morning daylight to run over the missed bits and smudges/ fingerprints where someone has done a "is it still wet?".

I did need two coats on my son's bedroom walls - they are black - but I probably wouldn't have it I'd taken more care for the first coat.

If you use cheap paint you will almost certainly need a second coat.

Maybe, it depends on the paint and (I have found at least) the application method. I find paint pads leave a lot of paint on the wall and therefore often don't need a second coat if you use quality paint.

I recently tried the B&Q colours range of paint. It's so thick it's like colouring the wall with a crayon! You really do only need one coat with it.

If after one coat there are just a couple of little bits missed don't do a complete second coat just touch up. I find the best way to approach that job is to get a pack of small postit notes and, over the course of a week or so, stick a note everywhere that you need to touch up. It might sound like I'm mad but I never seem to get all the bits that need touching up first time round if I don't use the postit notes.