What does "snappy number" mean in Jazz?

Solution 1:

As has been pointed out in the comments, in the musical context, a number is a piece of music.

number

5 b (1) : a musical, theatrical, or literary selection or production
// The actors broke into a song and dance number.
(2) : routine, act
// The comedian's number had the audience laughing out loud.

https://www.merriam-webster.com/dictionary/number

Snappy is generally not a genre of music. It's more commonly a mood.

snappy

2 b : marked by vigor or liveliness

https://www.merriam-webster.com/dictionary/snappy

If we were talking about classical music, instead of jazz, we might use the term vivace (lively) in place of snappy.

In the quotation, Copland does seem to be using the term more broadly, and more or less as a genre description. He describes two modes of jazz: blues (down tempo) and snappy (up tempo).

This is not the way we would describe jazz today. Jazz and the blues are often grouped separately, and in any case, there are up tempo blues tunes and slow, non-blues jazz tunes.