Twice a Week
And how many times a week do you think one would have to play in order to accumulate 'at least 10 hours'? Do you consider playing for an hour in one casino and then walking to another to play a second hour playing once a week or twice a week? I would consider playing five hours per day for two days in a week to be playing twice a week and, at the same time, playing at least 10 hours.
Since you refused to accept the short (and, as it happens, relatively accurate) answer of 8,750, you'll have to be a little more specific in your criteria of what constitutes a counter. There are estimated to be 8,750 counters active in the United States today (or at least there were as of two years ago). Are you trying to guess at the impact counters have on casinos and then laugh at their foolish countermeasures? Well, guess what, it's been done to death.
One more thing: there are a lot of players out there who cannot be bothered with participating in internet blackjack sites. Based on my own experience in the world of gambling, I imagine that not 1 in 5 serious players visits bj21.com or any other blackjack site with any kind of regularity. Maybe your initial assumption that the traffic on bj21.com somehow represents a large proportion of the card counting community was made in error.