>>Currently ODP has more than 50,000 editor, do you think they still need more editors?
This is not my opinion, this is the unanimous opinion of staff and meta-editors: "YES, ODP needs more editors!"
>>Is it difficult to be accepted as an editor by the ODP?
Depends. Is it difficult to look at a 50-page website, and say in 5-15 words what it contains? Is it difficult to use plain, clear English (or other languages in World categories), following style guidelines? Is it difficult to pick the one best category for most websites on some subject you're interested in? Is it difficult to take the time to find websites that would fit best in some particular category?
For the vast majority of people (80-90% is probably not a bad estimate), one or more of these things is difficult: and so editing would be difficult, and filling out an application (partly consisting of samples of editing work) would be difficult. But if you find all of these things easy -- or are willing to work hard on the ones that are hard for you -- then it probably won't be too hard to get accepted as an editor.