What is the minimum number of individuals per group required for ADMIXTURE analysis?
The following paper provides an in-depth exploration of factors affecting power of admixture analyses under a number of different models. Although the authors state they "provide the open-source R software AdmixPower, to perform power and sample size analysis", in fact it seems the software is not available.

AdmixPower: Statistical Power and Sample Size Estimation for Mapping Genetic Loci in Admixed Populations

Tool availability:

https://research.cchmc.org/mershalab/Tools.html

Site asks for a password, apparently there is no option to sign up.