One can will be fine, i've done my grille and the fog surrounds twice and I still have half left.
In relation to the gloss - It would look better, but I'm not overly bothered about that. It's been on for about 2 months now, and it's not peeled at all, which was my main concern. I've read that you must not clean the car for a week and not to drive it for a couple of days as the plastidip is still 'sticky/soft'. I stilll drove it though and didn't experience any problems.