Dolby Atmos adapts to the actual number of speakers available. The sound is not transferred as channels, but as "objects" placed in 3D space. I believe the limit is 128 concurrent "objects", most of them can be placed anywhere in 3D space by the recording engineers. The decoder (basically a complicated computer program) in your playback equipment will do its best to map these objects to the available speakers, up to 34 for home use. But if fewer speakers are available, it will use whatever there is. The placement is done by playing back the sound in several speakers at the same time, perhaps a little stronger in one and weaker in a few others, all in order to allow your ears to hear a 3D space.
One example of an "object" might be a helicopter sound moving around in the space above your head. Once the helicopter has disappeared from the sound scene, the same "object" may instead be a seagull or whatever. If your playback environment has speakers above your head the playback equipment will "move" the sound around including above your head. The more speaker channels your playback equipment has, the better the resolution can be, ie you might be able to better pinpoint the sound in 3D space.
It seems like today many home systems are sold as 7.1.2, that is 9 speakers and one subwoofer. And as noted above, the decoder part of Dolby Atmos will adapt to what you have at home. Generally there is only one subwoofer used in home use -- the ear cannot place low frequencies left/right or up/down so the ear simply hears a sound with undefined place. To get the best effect from the playback system, it can be critical where the speakers are placed.
How many concurrent "objects" you actually get when getting a media may be something completely different. You have to read the really fine print on the media to know the exakt details. Or perhaps not? As this is consumer marketing, expect the sellers to tell only part of the truth, not the whole truth and not necessarily exact either. History has loads of examples. I would expect using the Dolby Atmos brand to require at least a certain amount of 3D spatial content in the media, but how it will sound in your home is not guaranteed. I would expect re-releases to have less quality in the 3D information compared to newer stuff, but I was born sceptical so YMMV.
