A number of things play a part in getting an account manager on your account. Firstly, you'll have different levels of reps, ranging for Marketing Experts to Client Partners, which all vary on the clients you work with, your account best practices, policy violations, product adoption and mainly the amount of spend you put through through Facebook. There isn't a fixed way of it, generally, they will reach out to you via your Business Manager to say you're eligible for extra support.