There are currently - at the time of this post - 57 merit sources with a total merit generation of up to 11975 sMerit per 30 days. Those are the merits generated by sources only, and given that a member can send half the merits he receives and imagining the best case scenario where every member sends all the merits he receives, the total amount of merits generated by the system would double to 23950 sMerit per 30 days. So, roughly 24k sMerit per month.
I don't know if it is as a result of this thread or not but it looks like theymos agreed that there wasn't enough Merit. There has been an update to the Merit Sources.
Merit sources
There are 57 merit sources with a total merit generation of up to 14750 sMerit per 30 days
The same number of sources but more sMerit.