OP is asking for Merit, not sMerit. The total Merit supply can be summed up from the data in Open scraped data of all the users - SQL Lite DB - 2.481.270 users.

As for sMerits: theymos airdropped about 600k in January, 300k sMerit has been sent, creating another 150k sMerit in the process, and sources have added at most 200k sMerit. Many accounts have been banned, which reduces the total supply again.
I would estimate around 4-500k sMerit to be available at the moment. Merit isn't scarce, good posts are!