Yes 1 post is 1 activity, but you can only get 14 activity every two weeks every post more than 14 in this period won't raise your activity, which doesn't mean you should stop posting for example if you're discussing something in some very hot topic. That means you can post every day and get 1 activity a day or you can post 14 times in one day, come back in two weeks and do it again, though I don't see a point to this 
It's all very well explained in that thread about the ranks people showed you.
If i done the 28 act. today and after 2 weeks i post 14 more activity will i rank up?It's all very well explained in that thread about the ranks people showed you.
Ya you will rank up only on the said activity points after only the rank changes, i think the link given gives you full details about how many activity points needed for each rank so on every 14 days you will get 14 points so just add this and calculate it that how much activity points needed for each rank to go up and you have to post according to it. be active on the forum itself will give you more information about it.