LD50 in humans for MDMA is 10-20mg/kg according to this source: http://thedea.org/technicalFAQ.html
The mean being 15mg/kg which is the number I will use.
If you weigh 140 pounds, or 63.5kg then you have a 50% chance of death at 952.5mg.
If your average pills contained 70mg it would mean about 13 and a half of them.
You can do the calculations yourself with weight (kg) * 15 divided by 70mg per pill or more if you think your pills are stronger. The average is said to be 70mg though some could still contain up to 100mg.
http://www.teaching-english-in-japan...version/pounds if you need to convert pounds to kg. (Damn americans)

jonny is right in saying that you cannot do mdma every day simply because your brain will eventually run out of serotonin...no matter how many more pills you try to take.