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Number of posts are not limited, regardless of your rank and whether you are a Copper Member or not. There is a cooldown time between posts in accordance to this formula, which is related to the Activity you have (the 360 figure is in seconds):
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waittime = 360;
if(activity >= 15)
waittime = (int)(90 - activity);
if(activity >= 60)
waittime=(int)(34.7586 - (0.0793103 * activity));
if(activity >= 100)
waittime = max((int)(14-(activity/50)), 4);
If someone has gathered 350 Activity in the last 6 to 7 months (in the maximum range than can be obtained in that period) and has earned 0 merits, it likely means that he not making contributions that are noticeable in any way, or that he is posting in sections that are extremely scarce on merit (*), or spam megathreads.
(*) i.e. Chinese, Romanian, Korean, Scandinavian (see https://public.tableau.com/shared/WHQRPTC37?:display_count=yes&:origin=viz_share_link)