The hard part of running many accounts is not the posting. Posting scales fine. What does not scale is the switching: logging out and back in, remembering which brand voice you are in, and the particular cold feeling of realising you may have just published a client's post to your own account.
The first thing to separate is identity from access. Sharing one login between several people is the arrangement that feels simplest on day one and causes every problem afterwards. Nobody can tell who published what, the password never gets rotated because rotating it means messaging everyone, and someone leaving the team turns into an afternoon of changing credentials across every platform. Access that belongs to a person, and can be removed without touching anyone else, is worth setting up before you need it rather than during a handover.
Then keep one calendar, not one per account. The instinct is to give each brand or client its own plan, which is tidy inside each account and useless across them. A single view is the only place you can see that three accounts are all publishing on Tuesday morning and nothing goes out on Thursday, or that a campaign you promised is running in one place and not another.
Name things so that a stranger could pick them up. Anyone managing more than a handful of accounts eventually goes on holiday. Consistent naming for drafts, campaigns and recurring posts is what makes it possible for someone else to step in without a briefing call, and it costs nothing to agree on at the start.
Expect approvals to scale worse than posting. Publishing twenty posts is barely more work than publishing five. Getting twenty posts approved by four different people is a different problem entirely, and it is usually the thing that actually delays a calendar. Batch reviews rather than sending things one at a time, give reviewers a deadline, and agree in advance what happens when the deadline passes.
Audit access on a schedule, not when something goes wrong. Once a quarter, look at every account and ask who can post to it and whether they still should. Old team members, former agencies and a tool someone trialled once tend to keep their connections long after the relationship ended. The review takes twenty minutes and is the single cheapest security thing a small team can do.
None of this is exotic. It is four decisions: access belongs to people, one calendar for everything, names a stranger can follow, and a standing review. Make them early and the tenth account costs about as much attention as the third.