MDMA, like most perception-altering substances, has an 'ideal' dosage at which the effect is most enjoyable. That dosage seems to be a little less than 1mg per lb (or 2mg per kg). For most adults, this means a dose in the range of 100mg-125mg.
Uh...I bought this pill, how do I know how many mg it is?
Unfortunately, users of MDMA are forced by unreasonable laws to purchase the substance illegally. This makes it very difficult for the average person to know how strong their pill is. Generally speaking, though, most pills weigh in at a little under 100mg. Larger pills may contain 100mg or more, but these are less common.
So how many pills should I take?
One is always a good number. If you find it to be a little weak, you might try one and a half (bite the second one in half). IT IS HIGHLY RECOMMENDED THAT YOU NEVER TAKE MORE THAN TWO. In most cases a dosage this strong can incapacitate you (as you are 'rolling too hard'), and might even be dangerous (see Safety Concerns). Taking too much can also cause your serotonin to be completely depleted even before the MDMA has run its course which can cause depression and/or a severe "come down" feeling.
One or two pills used to do the trick for me, but lately I've needed more and more to roll.
Take a break! Your serotonin receptors have been bombarded from using MDMA too often. They will return, but you have to give them time. If you don't, you will find yourself continually needed more and more pills, until finally they simply do not work at all.