|Hi there, welcome to the board. I'll take your questions one at a time to try and make it easier to follow (I tend to babble! LOL!).
-how many diapers, liners, and covers you need for one kid
Diapersasically depends on how often you want to wash. Figure that a newborn will go through 10-12 diapers a day, gradually that will go down to 6-8 per day for an older toddler. So if you want to wash every two days you'll need 20-24 diapers for a newborn and 12-16 for a toddler. You still have to figure though that your washing machine can only wash so many diapers at a time so there's no point in buying like 70 diapers and hoping to wash only once a week. Two dozen is about average with some making due with less and others indulging in more. Consider also how you plan to dry the diapers, if you plan on line drying then you'll need a few more to get your through to when they're all dry.
Covers: An average amount is 4-6 but you can get by with as little as 2 (one on baby and one you wash by hand and let dry - they usually hang dry in just a few hours). If you plan on washing with clothing and don't wash clothing very often then you may want a couple more.
Liners: If you have a newborn or young infant you won't need these at all. Usually you only need liners for nighttime when baby stops pooping at night and you want to extend the time between changes and therefore need more absorbancy. Figure you'll need 1-2 liners per night (extremely heavy wetters may use 3 per night but that's unusual) at that point. In some cases people using the One-Size and that have very heavy wetting toddlers use a liner in each diaper. This is the exception though and is usually only with older toddlers.
-How many times do you wash the covers a day
Everyone has their own system but you definetly don't need to change the cover with every diaper. Some use one cover all day and night long, changing in the morning. Others put a fresh one on at night. Others give the cover a quick rinse with every change and rotate 2-3 covers throughout the day, only washing every few days. You'll figure it out for yourself but basically you only need to change when it's smelly or of course if it's soiled.
-Also how does one-size work for newborns -- are they small enough
Short answer is yes, but it does depend on your perspective. If all you care about is function and saving money, then the One-Size are perfect. They won't leak, even for small newborns and you'll only ever have to buy one set of diapers. If you think that bulk will bother you then the fit of the One-Size may bother you. They work fine but they are quite bulky at first and although this doesn't bother the baby at all (honestly, they don't care and it's actually good for their hip development) it does bother some parents.
-Do you need pins/snappies for ME diapers?
Nope, they fasten with snaps.
I think I covered the liners above but I know it can be confusing so don't hesitate to ask any questions about them or anything else.