How many Champions I need to per lvl?
I don’t think anyone can really answer your question as the amount you need increases for each level and it depends if you get gold, silver or bronze. Plus it matters a bit if you are in a level appropriate zone or not.
This site here shows how much exp you need for each level. http://wiki.guildwars2.com/wiki/Experience#Total_experience_gain_per_level
You can kill a champ or two, see how much exp you get and calculate it from there.
There is a curve to leveling in the early levels, but it does even out eventually (time-wise). I guess you would have to find out how much XP you get from each champion, and divide the amount of XP needed for your next level to find out how many champions you needed for that particular level.
Here are some charts that may help you: http://wiki.guildwars2.com/wiki/Xp