Fourty to fifty directory submission is enough for new website on daily basis and it's completely depends on your keyword that how much, it is competitive. If it's more competitive then you should need to do more submission to get listed quickly in google.
for getting web site in top 50 position , it depends on keyword competitor and other metric for getting rank
keywords easy: just on-page is on the top 50 Keywords Medium: 10 to 100 directories Keywords difficult: from 100 directories and more
Total 500 approved directories are enough. You have to submit in Do follow directories with high quality and submit in PR websites(Minimum 3 PR).
If there was a limit then every one did this. there is not special limit . Get few but genuine backlinks.
It depends on the keywords you are targeting mate. Sometimes only utilizing general web directories to gain inbound links and to rank for keywords is not enough. You also need links coming from different types of pages, like blogs, articles, bookmarks, citations and many others.
Directory submission is a long term seo so to get fast google ranking, you need to do bookmakring, blog comments, article posting and press release .
There is no limit to directory submission, you can submit as much as you can, start with high pr directories( Use variation in title). But i would suggest you to do all other off page activities for targeted keywords to get quick result.