No, I believe each merit source have differing number of sMerit generation per month. There are some merit sources who generate less sMerits while there are others who generate more sMerits. Only Theymos and those merit sources themselves know that how many sMerits they get per month.
Just chiming in here as a merit source (even though OP's already got his answer): yes, not all merit sources get the same monthly allocation of sMerits. Last I knew I was tied with one or more sources for the highest amount, though that might not be the case anymore. Either way, it was because I kept requesting a bigger stash since I kept running low each month and monthly allocation isn't for bragging rights and isn't why I mentioned it.
In fact, I could probably do with a
lower number of sMerits each month right now as I've ended my post history review offer. Maybe Theymos could give me 400/month with the remainder going to a new merit source, and preferably one who could cover a local board. Theymos himself might not see this post, but perhaps a sympathetic moderator will. I'm still trying to be active, but I'm sure I've got too many sMerits to unload each month.
Being a MS....a job you both love and hate to have at times.