Ive seen 18 total enemies. 10 Aliens (do you even count eggs?), 6 Synthetic types (one holds two different weapons maybe seegsan drones counts as two? Riot and melee? I don't believe enemy turrents should be counted as an enemy), and 2 misc. (Queen and Facehugger).
2 days left and maybe 1 mystery alien and 2 miscellaneous that we haven't seen for a total of 20. I hope these will be officially unlocked on release date and that those people with hard copies that were shipped out have something locked and don't know it.
I think someone said that there might be an update patch on release day so i'm hoping these Twitch streamers know that there is something still unavailable to them.