I think do 50 submission in a day because it is feasible , and you not only have to submit on directories there are other things also to be done.
As mentioned above, use a variation of Titles, Descriptions and keywords and try and submit the relevany categories, try not to submit to exactly the same category to avoid spam. Submit as many as you like as the busier directories could take months to approve. Google does not add all of these as backlinks anyway. Good luck
No limit. You can submit to as many directories as you can... most of the directories can't approve you instantly.
Are you asking how many you should submit too, or how many of the submissions to your directory you should review?