How much power do you need to claim a faction land in Minecraft?

At least one person. The more people, the more land

I'm guessing you mean claim another faction's land?

If that's the case then it actually depends on their faction power. Meaning that if their FP is 10 than the maximum number of chunks they can claim is 10. If their power later drops to 8 than your faction or any other can claim to of their chunks.

If you're talking about claiming the wilderness than all you need is 1 FP for 1 chunk of land