If you check the various wekis, websites, etc, you'll get a bunch of different answers. Some count all the ones not yet shipped; others rely on bad data, and others are merely propaganda.
The number they eventually agreed on (as a guesstimate) was 1.8 million XO laptops were actually in the hands of stundents. I'd say that is a lot, considering that it has no marketing budget, brand presence, and the fact these buggers cost around $500 each to deploy.
If you compare it to smartphones, it's not a lot. But if you compare the XO figures to another niche product like ereaders, then 1.8 million users in only a few years is quite a lot. It's actually more than some ereader makers have accomplished.