If you go to the bottom of the Wyeast page here
There is a good graphic on how you can estimate the number of cells in your slurry. you can do this with a 50ml tube, by mixing up your slurry and filling the tube with 50ml of the slurry, then letting it settle.
also you can weigh the yeast suspension to get an estimate of the total yeast in solution. but I don't have the numbers here with me, but if someone has access to the Yeast book, they might post it here for you.