How do you model marketing channel saturation?

This is more of a mid-game question for seasoned marketers / growth hackers:

When building a model for evaluating various marketing channel investments, how do you model saturation of a channel specifically regarding the product?

Say, (hypothetical example) you're a marketplace for business coaches, and are evaluating influencer marketing: how do you estimate the number of potential influencers, conversion rates thereof, and ROI as a function of incremental spending increase?

Would love to hear specific case studies around channel scoping estimations, thanks!