There is a limit on the number of videos that you can favorite on YouTube is 650. However, there are some YouTube users who exceed that limit and go up to 800 to 900 favorites. Visit youtube.com for more details.
Well I have 1,706 favorite videos. When i had like 1,300, Youtube didn't allowed me to have more but i deleted 5-6 videos and i could add like 30 more vid. Then I also deleted 5-6 and now I add as much as i want without deleting any :)