I don't know the exact amount now, but dooglus has said in the past that Just-Dice usually stakes an average of 1200 clams per day, but since the site have invested almost the same percentage (over 75% of all Clams in circulation) as in the past I think they should not be a huge difference in the amount that is staked daily on Just-Dice now, recently dooglus also told that Just-Dice staking wallet has show some strange variance that has made the performance drop a little bit so I guess that must put the average a little under the 1200 Clams per day.

As external data you may be interested, Clam produce 1 stake per minute with 1 Clam as Block Reward that give a total of 1440 Clams per day to the Clam Blockchain, as you can see Just-Dice is a huge staking machine.