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)
Your activity is equal to the lowest out of the following two numbers:
- The number of two week periods you have posted in multiplied by 14
- The number of posts you have made
Let's assume you have made 30 posts in a two week period. Your activity is now the lowest between (1*14) and 30, and so will be 14.
In the next two week period, you make 1 post. Your activity is now the lowest between (2*14) and 31, and so will be 28. You have gained 14 activity despite only making 1 post.
In the next two week period, again you make only 1 post. Your activity is now the lowest between (3*14) and 32, and so will be 32. You have gained 4 activity despite only making 1 post. Your maximum allowed activity now will be 42, which you could gain if you made 10 more posts.
Essentially, every two week period you post in adds 14 to your maximum allowed activity, even if you only make a single post. You can then "collect" that potential activity by posting more in subsequent two week periods, or from additional uncounted posts from previous two week periods.