https://wiki.project1999.com/Faction gives you some idea of the numbers of kills that may be involved, although even it's incorrect (some levels can take more kills than the chart on that page suggests).
Also, don't be confused by the wiki faction numbers: they are lies (the +/- part is correct, but the specific numbers aren't)! While I'm all but certain a Vindi kill will give you significantly more faction than killing a random giant, the exact numbers in the wiki are generally way off, and many things that claim to give +5, +10, etc. actually only give +1.