Sharing equipment/space in the warmup room is essentially a necessity unless you're lifting at larger national/international competitions. The last competition I did, there were 4 warmup platforms, and I think 11-12 lifters in my session, and only once have I been to a competition where the warmup area was sufficiently equipped for everyone to have their own warmup platform. Like NewWorldMan said, pair up with teammates or people lifting similar weight. Shouldn't be a big issue.
As far as the warmup attempts, they should be whatever the athlete feels they need to be comfortable in going out to the competition platform for their opening attempt, and that'll be a little different for everybody and something you should figure out in training. Some people need more/fewer reps, some people do doubles or triples up to a higher/lower weight, etc etc. As an extreme example, I remember watching an interview of Koklyaev after a meet and he said he did 4 warmup attempts for his clean and jerk, although he admitted he should have done more. As a specific example, for my last competition, before snatch I did:
general warmup (BW squats, leg swings, eagles/scorpions, played around with a broomstick/dowel, etc)
bar work (for me, generally power snatch, overhead squats, drop squats and full snatch)
Open at 125
As an alternative some people might prefer to double the 80 and 90 as well. Clean and jerk would be similar, minus the general bodyweight/mobility stuff (sometimes I skip the bar work in training if I'm moving directly from snatch to clean and jerk though). And as a final note on this I'm not a huge fan of working in percentages because I like loading in increments of 5 and 10, just makes life easier.
For determining opening attempts, I used to go with the best weight I'd hit in training over the 3-4 weeks prior and open with that, determining my 2nd and 3rd attempts based on how that one went. This was because I had a tendency to get a lot out of my taper, and hit a lot of PRs and generally lifted very well in competition. Then that stopped happening (had a long string of competitions where I went 2/6, 1/6, that sort of thing). So for my last competition I dropped 5kg off my training best leading into it, opened there, and it went much better (although there's a host of other factors that played into that as well). Think I'm probably going to stick with something akin to that for a while.
And as much as I think it's important to be able to gauge yourself after your opener and adjust your 2nd and 3rd attempts accordingly, I think it's also a good idea to have some sort of plan laid out as far as what you want to hit, and how your attempts are going to get you there. As an example, I have a dream of hitting 146/182 at my next competition. So if those were to be my 3rd attempts, I'd like to have hit at least 140-141 and 176-177 in training, and the attempts would probably look something like 135/140/146 and 170/176/182. Course I'd have to be a little less fat than at present, but I digress. As NewWorldMan said, it all depends on your goals for that competition.