Being an almost spinoff device, the Mini wouldn't have a version number as it is neither a predecessor nor successor of any model - thus eliminating any possibility it could be a ".5" version like the V4.5 and V5.5. Furthermore, the sequence only goes up to V9 as the rereleases of the Mini and the upcoming Original Tamagotchis will not continue the sequence as they already have their sequence "number". The Mini could fit into a sequence of order of release or perhaps even year of release (which found on the Tamagotchi Wiki).
Just for the record, the sequence by versions is V1 (noted to be implied by Jhud), V2, V3, V4, V4.5, V5, V5.5, V6 (Music Star), V7 (Tama-Go), V8 (Original Tamagotchi Friends), and V9 (Tamagotchi Friends Dream Town). There hasn't been an original English Tamagotchi to date and perhaps the sequence will be broken by a new version that doesn't follow the sequence and can't connect to either version of the Tamagotchi Friends.
Y'know, I wonder if I could freak out non-tama people by how I recounted that entire chronology from memory. I can even do the vintage one too @_@
Edit: Nope. I checked Project TamaShell and was going to put the Devilgotchi right after the Angelgotchi, even though I knew for a fact that the Devilgotchi earned its rarity due to being released so late in the vintage Tamagotchi life-span. Then it turns out Project TamaShell got it wrong as the Tamagotchi Wiki claims that the sequence went Originals, Mothra, Angel, Genjintchi, etc. So many tamas released in Japan in 1997 makes me realized I would be broke if I grew up there and then.