So....to gain the maximum amount of activity per week (14), we have to make 14 posts? If we create less posts, the activity gained will be less, but if we create more posts, the activity won't exceed 14? I think this is how it works, am I missing something?
The only bit you are missing is that it only takes one post in any 14 day period for that period to be considered active. Then if later on you make more posts your activity will increase up to a maximum of 14 for each period you were active. So there is need to post too much when you start out, just make sure you post at least once every 2 weeks.
Is it okay to comment on posts to gain activity and rank?
No, it is not. Only post if you have something constructive to say or you may get banned or tagged as a spammer.