Take theymo's words, the admin in charge and developer behind most of the forum's features (if not all).
The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every hour.
I underlined the answer to your question and to fix your wrong understanding regarding the matter. I'll take it more in a different and effective way to explain this to you, you need to atleast post one in the two-week period and the remaining activity will become a
potential activity.
Take this example, you posted 3 posts in that two-week period time hence you will gain 3 activity (in the fact that you don't have post counts in line to activity count but only those 3) which didn't reach the maximum 14 activity count hence you have 11 potential activities. Here's the catch, if you didn't post atleast one in that two-week period hence you will not gain any pontential activities and will not receive any activity count of course.
Don't get confused. Activity updates every hour, it means that if you have potential activities then it will get counted even if the two-week period time hasn't yet to come i.e. in hourly basis. While two-week period time, from the word itself, this takes place every 2 weeks or 14 days with an addition of 14 activity count or potential activities i.e. if you post atleast one in the given time frame.