The problem is: You are not really able to count them, because the texture slots get filled with model (&other) textures dynamically during in-game play and I am not sure if they ever get released, when the caching of models option is turned on.
So the longer you play, the more texture slots will get filled. Model caching should be a feature, but unfortunately in combination with MAX_GL_TEXTURES it acts like a bug.
Another funny thing is that we have virtual in-game limit of 1024 models per map (MAX_MAP_MODELS), which alone would use up ALL of our texture slots when really used... You can turn off model caching to free a lot of the texture slots...
So to avoid bugs during play in-game (also with other maps), we should rework model caching and limit the amount of cached models to a specified number if MAX_GL_TEXTURES = 1024 is to be set in stone.
Edit: The MAX_MAP_MODELS stuff is not true, because it stands for the total number of map models allowed, not the number of different models, so forget that one, please...