Directory submission may be very useful. In general it gives permanent Do Follow back links. Directory Submission may take times to get approval but it gives us back links. For getting best result you have to approach for Direct Submission.
50 directories submission should be done in one day. keywords should be 4 to 6. You should have to done a 1500 directories submission for a new websites. You will start getting emails after 3 to 3 weeks for approval/rejection from different websites.
You also see the approval websites links on backlinkwatch.com.
" The main Things in Categories" Be careful when you are going to select categories.
Personally I don't see the benefit of directory submissions nowadays. It's always important to gain backlinks, but I would focus on getting links from quality, relevant pages across the web. Not just listings in free-for-all directories.
there is no such limits made on how many but, if you want to get the maximum benefit you should coose only the high quality directories. Choosing the quality directories is the key of getting the maximum benefit. Use the appropriate title and description.