You don't know how many you get from each (only way for a member to know is to have the number of your points, then when you get a new rep you subtract your new rep from your old one and then you will find how many you get). The only other way is to look in the Admin CP and see what the rep was.
It is determined by a few things. Join date, post count and their own rep count.
Join Date: For every 100 days some one has been a member of PC they will get one higher rep power (rep power is how much you can give or take from a member). So if you been here 300 days then you will be able to give or take 3 rep points.
Post Count: For every 500 posts you will be able to give or take a rep point. But even if you have been on PC a year or longer you have to have atleat 50 posts before you can give or take points (so even if some one has been a member for 100 days but have no posts the box will show up grey).
Own Rep Points: Even your own rep adds to what you can give or take to other members. For every 30 rep points you will get one higher rep power. So lets say you have 60 rep for yourself. Then you will be able to take or give 2 points on that member. But you have to have atleast 10 rep points or it will show up grey like the 50 posts thing.
And remember all them things add up to a higher rep power. Like if you have been here 100 days, have over 500 posts and your own rep is 30 then you will be able to give or take 3 rep points.
And to your last question about grey rep boxes.