You can earn max 1 activity point daily. 1 written post brings 1 activity point, and everything is calculated for 14 days.
Potentially, a user can earn more than 1 activity point per day. If you make 6 posts in one day within an activity period you earn 6 activity points, but as you pointed out it is a maximum of 14 points in 14 days. The only outlier is when a new user opens an account close to the end of one activity period and can earn 28 points within the next 14 days + the number of days left for the activity period they joined in.