I would recommend between 10-12 to start off with. Some people manage fine with that many. Other people like to have more. It also depends on how often you want to wash. I liked to have plenty so I didn't need to wash every day. 18-20 maybe a more realistic number. Remember you don't need to buy them all at once.
Yes you can. For night I would recommend a combination of hemp and microfibre.
No you don't soak modern cloth pocket nappies so you do not have to worry about the expense of napisan. The sun is a great way of removing stains.
No you don't. The only time you need to use a nappy liner is when you are using a barrier cream. The nappy liner will protect the nappy as barrier creams can leave a residue on the nappy. Some people like to use nappy liners all of the time and some people don't. Its a personal choice.
No you don't need to use a cover. The outside of the nappy is made of PUL which is a waterproof fabric so a cover is not required.