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.