We usually take any amount that would give us statistically significant metrics, so this differs between the influencers but is usually more than 10k followers. We closely look at every follower and determine the gender, age, and location based on the profile metrics and post captions. We don't get this data from every follower, only enough to be statistically significant.
A post’s reach is calculated based on the engagement of each post, the quality score, and the audience data (if available). Besides that, we made a lot of testing and comparison to determine what influences these values and how to get closer to real values.
Still, our data isn’t 100% accurate, and there might be discrepancies in the real values. One big factor is that we look mainly at the active and engaging audience, and the provided data by the platforms takes the full data of the audience into account. In conclusion, our data gives excellent guidance on the relevant metrics.
We always recommend looking at profiles and reading the comments under posts to get a feeling of the community of the given influencer since this gives insights that algorithms or calculations can't determine.