And if I could so impose on those gifted and experienced writers who decide to answer and/or comment on @Kit Caruthers' thread, what kind of market is there for novellas?
They're usually separated by word count rather than page number, which varies based on formatting. Even then, the distinction is rather arbitrary. Different places have different standards. The SFFWA considers anything between 17,500 words and 40,000 words to be a novella.
Assuming 250 words per page, a 20 page story is about 5K words - definitely short story territory. In terms of a market? As with so many other things, it depends on genre. There's a pretty solid erotica and romance market in novellas, but less so in other genres. It may be easier to sell self-pubbed novellas (at an appropriately lower price) than self-pubbed novels, as a reader may be more willing to take a chance on a 99 cent novella rather than a $3.99 novel.