I guess we can safely guess 65-70 techs total? There were ~85 in vanilla Civ 4, if I can count accurately. I guess if we add the social policies (of which there are 40, 30 available at a time) there are actually more "advances" to make, but there are less techs.
If you look at the tech tree that i posted, it makes 70 known techs, and it still continue to the right.