North American Butterfly Association
The North American Butterfly Association (NABA) is, by far, the largest group of people in North America (Canada, United States, and Mexico) interested in butterflies.
Here are some quotes from the FAQ:
How many kinds of butterflies are there?
There are approximately 20,000 species of butterflies in the world. About 725 species have occurred in North American north of Mexico, with about 575 of these occurring regularly in the lower 48 states of the United States, and with about 275 species occurring regularly in Canada. Roughly 2000 species are found in Mexico.
How many kinds of butterflies can I find near where I live?
In most parts of the United States, you can find roughly 100 species of butterflies near your home. The number is higher in the Rio Grande Valley and some parts of the West, somewhat less in New England. As one goes northward into Canada the number decreases, while as one goes southward into Mexico the number greatly increases.
How long does a butterfly live?
An adult butterfly probably has an average life-span of approximately one month. In the wild, most butterflies' lives are shorter than this because of the dangers provided by predators, disease, and large objects, such as automobiles. The smallest butterflies may live only a week or so, while a few butterflies, such as Monarchs, Mourning Cloaks and tropical heliconians, can live up to nine months.