|I can't help you much with the roughness, although I have read on the forum that washing with vinegar can help mineral breakdown and make diapers softer, but I don't have first-hand experience.
As for quantity, I can tell you that in the beginning, I used between 12 and 14 diapers a day. It goes down a lot with time (my 2-year-old uses maybe 7-8 a day). With 17, you will likely be fine if you wash every day, but it may be stretching it if you want to wash every 2 days, especially since I assume that flats don't absorb as much as a OS does. Although in the beginning it's rather pointless because babies poop so often you have to change them all the time anyway.
Also, keep in mind that flat diapers will not be representative of how awesome cloth diapering would be with ME OS diapers... They are so easy to use, and they never leak! Although again, with newborns, many people do use flat diapers successfully...
In terms of covers, I preferred having 3 or 4 so I could wash them with laundry, but they dry really fast, so you could have 2 and hand-wash them. But I have no experience with flats and I would suspect they may be more prone to leaking on the cover, which would mean you may be happier with 4 than with 2.
Anyway, people who have more experience will likely give you more useful info! I just wanted to congratulate you for trying cloth, hope you like it as much as we crazy forum-addicted-people do ;-)