It's mostly for those who'll turn to the novel after watching S1 of anime.
https://anilist.co/anime/127090/Isekai-Meikyuu-de-Harem-wo/
For some reason Opening of S1 contains footage with full harem. Leaving aside the decision-making of production team, I want to let you know when we meet all girls in the story, at least in some way. :)
The numbering is from WN (Web Novel) that currently has 225 chapters of which all are translated. WN is on Hiatus.
There's also a Light Novel, but with 12 volumes released, the translation to English is around Vol.5 atm, so it won't serve as a good measure of Harem acquisition timeline, I think. Still, I'll add LN volume number for the slaves present in translated volumes (as far as I know). :)
As the title suggests, all party members/harem members are MC's (Michio) slaves.
Screenshot with everyone from the opening of Anime S1 under a spoiler tag.

In order of acquisition:
- [Roxanne](https://anilist.co/character/141842/Roxanne ) (Wolf Tribe)
WN Vol.1 - Ch.28 / LN Vol.1 / Anime S1 Ep.03
- Sherry (Dwarf)
WN Vol.2 - Ch.48 / LN Vol.4 / Anime S1 Ep.11
- Miria (Cat Tribe)
WN Vol.3 - Ch.82
- Vesta (Dragon Tribe)
WN Vol.5 - Ch.122
- Rutina (Elf)
WN Vol.7 - Ch.190
There's no confirmation for this ( -> ), but it's probably safe to say that he won't buy anyone else since the party can have maximum 6 members (MC + 5 slaves, in this case).