Hi guys,
There isn't actually a 'limit' to speak of. You can put in 20 if you like. However, use the preview tool (by clicking the magnifier) to see what will come up for the keywords you input. You will find that, once you have any more than, say, two keywords, Google will only pull ads for the 'stronger' of the terms. If you put: "cars, motorbikes, trucks, pickups, bicycles, automobiles" - you will not receive 2 ads from Google for each of these keywords. In fact, you'll find that if you put cars, motorbikes, trucks, you will get just the same ads. This is because nobody on Google is bidding on these combinations of keywords. This is different for phrases, such as DVD players where Google can read this as one phrase rather than two separate words.
I would really try out the magnifier and see how you get the best ads. I personally feel that you should set just one well-researched keyword. If putting 'cars' doesn't get the results you wish, don't enter cars, automobiles, autos, but try a more precise keywords, such as "car parts" or "used cars". This will help to get the most out of the domains.
I hope this helps and that you all have a great New Year.
Ed