About 22 Million in March 2007 so lets be real kind and say Apple increased their userbase by 50% since then as this is the MacRumors Forum

. With Vista's sales numbers increasing (it increased by another 40M in only 3 months from Apr to Jul) and the trend showing takeup of x64 increasing (from 3% to 20% in the same time period) how long do you think it will take x64 Vista to overtake that number?