Most home and portable pianos are equipped with an audio system – an amplifier and speakers - that makes the sound audible at various volumes. Stage pianos, generally, are not provided with speakers and will usually have to be connected to a PA (public address) system or external amp. The minimum number of speakers you can get is 2, one left, one right.
The more inexpensive pianos will use a set of “full-range” speakers that can cover between 20Hz and 20kHz. These speakers usually do well in the mid-range but struggle to render highs and lows at high quality, that’s why more upper-end pianos use at least 3 speakers: one optimized for the bass, one for the mid-range and one treble on for the high end. Low frequencies tend to radiate in all directions, while the higher the frequencies the more directional they are, so speaker placement is an essential consideration in design these pianos. Most of the low-end and mid-frequency speakers on digital pianos are located below the keyboard because there’s plenty of room there. The more directional nature of the high frequencies requires pointing the tweeters directly at the player’s head, usually from somewhere on the instrument’s control panel.
As aforementioned, digital pianos generally feature 88 keys, and there are more compact versions as well as cabinet, or home piano, designed to look like an acoustic piano. The advantage of this type of pianos is the quality: being bigger, they have a bigger speaker system resulting in better sound quality.
Premium digital pianos will feature more speakers to help the piano feel the room just like an acoustic one.