How many accounts can you keep on one phone without losing coverage?

The device answer above is the usual one and it holds. The part of your own post that hasn't been picked up is the last sentence, that all the accounts carry the same type of content, because that's a separate problem from device linking.

Accounts in the same niche get shown to the same starting audience. Two of them don't reach twice as many people, they split the same viewers, and each one then earns a weaker early signal than one account would have earned alone. So coverage can drop with no penalty applied at all. If the accounts are meant to add up rather than divide, what they need is different audiences, not just different logins.

Best,
Floqal

around 3 tusually seems manageable but once you go beyond that the activity patterns can become more noticeable. if youre scaling up its better to keep things natural instead of doing everything at once.