ive not worked in a profesional environment (=paid), but i can answer most of these with my modding/indy game experience. i'm also throwing what knowledge i have of the industry in here.
First I was wondering if there are any concept sketches to find (on the web) op perhaps to expected. (and where they are to be found, of course)
not quite sure what you are asking here. companies usually put up concepts on their websites as promotional material. conceptart.org is also a must-visit, many people (both in concept art and in all other art industries) post their work on the site.
Secondly, I'm interested in the process of designing such a huge world and all the characters in it. Where do you start?
whereever the design document/art lead tell you to
in my experience, you usually start at larger, more influencial items (eg. races or regions) and atmospheric insperation pieces, then narrow down and do specifics.
How many people are working on such a project?
depends on the size of the company and the project. in the beginning there might be more, towards the end of the project less.
how many time/sketches does it take to "design" a creature or city?
depends on what youre designing. sometimes the first sketch is perfect, sometimes it takes a dozen to get it right. often times the artists do brainstorming sessions, picking parts of each drawing to merge into a final. cities take more because each element (building, flag, lamp post, etc.) needs to be designed individually. also, one sketch often means doing 4-5 separate drawings so that each side/function/variant of the object can be seen.
time depends on your own speed and on the requirements. beth's concept sketches on the front page look to be just pencil, other companies use full digital. some assignments take more time, some less.
Last, I'd like to know what you are looking for in a concept artist (My drawing skills are by far not good enough to do professional work, but it's always nice to have a goal to work to)
the first thing
I look for in an artist is creativity. i dont care if youre freaking davinci, if you cant have a good, non-generic idea i dont need you on my team. however, art skills are also pretty important - if you cant render your idea its not very useful, now is it? from what ive seen of other people's paid art, you dont have to be perfect, but you do have to know how to get your idea across. dedication and persistence is also very important, as is a varied portfolio (so not just bikini clad fantasy women or shiny steampunk bikes, for example).
hope that helps