They are everywhere in Alberobello, the Trulli. The Trulli phenomenon could be considered as one big tourist trap, by which the livelihoods of a whole town are supported. But I found them too picturesque and too much fun for that. The Aja Piccola area is less commercialized than the Monti, and therefore generates a more authentic feeling.
You can spend a few hours roaming around both quarters. Only more time is needed when you want to make a study out of the small differences between all Trulli (don't bother, someone has done that already and came up with 5 categories).
There are some exceptional Trulli in Alberobello. For example the two-storeyed Trullo Sovrano. It's quite spacious inside: it has several rooms downstairs, a garden and a second floor to house guests. Another, with more subtle differences compared to the normal ones, is the Trullo Siamese. Siamese twins are said to have lived here, or maybe it's just two Trulli under one roof.