yes off course you can use multiple key words to use in same ad group, you can use many ad groups too ..
Yes, you can use as may keywords you want in your campaign if you can manage them. Even you can use same keywords with different match type in same adgroup. If you are using same keywords in 2 different Campaign the keyword getting best ad position will be pickup by adwords to show ads.
Yes, It is possible to use multiple keywords in a campaign but it will be better if you use 20 to 30 keywords in each campaign groups.
As all the responses have stated you can use up to I think 1 million keywords but I agree with an earlier post that you should not usually use more than 20 keywords in any particular ad group. Ad groups are used to segment your keywords into smaller groups to keep them relevant to the ad and increase quality score. Remember, the more specific the keyword, the more relevant you can make your ad. I use maybe 20 phrase match keywords per ad group, maybe only 10 if they are broadmatch, and maybe as many as 30-50 if they are exact match keywords. While I have seen advertisers create an ad group for each keyword they use, I do not believe that is necessary, but by all means, if you have the time, the more relevant the ad the better.