My guess is 400k!
Pretty big!
I wonder how you came to that conclusion?
I think that the most accurate way of measuring it would be to make an poll to determine the average number of transactions that makes a unique user every month, and then to divide the number of transactions by this number. But it would be difficult to determine the average number of transaction per user. Should take 1 - 2k of significant people to answer.
Just a guess!