Don't worry about making friends, genuinely, everyone is in the same boat, so if you tag along with people you have met in the corridor for nights out, you will probably end up being close with them! In my deluxe room, I am in Burbage, (the building name), and there are 2 deluxes and 3 ensuites in our flat. I think that is the minimum. Don't worry though, our building has about 45 other people, and that's quite small for a building! My friend is in Rivelin and she has a deluxe, but she has 2 deluxes and 5 ensuites in her flat, so really it is the luck of the draw. If you would prefer more in your flat, you can specify on the accommodation application (by saying in the additional information box that 'you'd prefer a flat with many people'), although, I would just personally leave it to chance, as the likelihood is that you will be put there, and apparently my flatmate's living room and kitchen are the same size as our kitchen and living room (despite 2 more people living there), so there may well be less space. Also, just because you are in a flat of 7, it doesn't mean your flatmates will necessarily be amazing, you could have people get on with you more in a smaller flat (so I'd leave it to chance, but it is up to you!).
Sheffield is the safest city in the UK regarding crime, so nobody really is involved in any. No cars have been stolen that I know of. Permits are £75/year I believe, but new rules insist you must only use the main car park at the weekend. The car park is quite busy then, but that's because visitors use it too. Alternatively, there is a HUGE road to park down, which people do, so you can always opt to do that for free. Spaces are limited, but saying that, I always get one (literally ALWAYS), so there is always space. It may well depend on if your year bring cars.
Failing that, there are car parks dotted around the campus which you can park on whenever, so you can always use them, it's just a few months ago, the university changed its policy on the main car park. They may change it back soon, who knows?