PostEasePresence without upkeep is worse than absence
A profile created on every platform and then left untouched for months reads worse to a visitor than not being on that platform at all. An obviously dormant account raises more doubt than a missing one. Being on fewer platforms, properly maintained, beats being on every platform in name only.
Start with where your audience already is
The right starting number depends on where the people you're trying to reach spend time, not on how many platforms exist. A business whose audience is concentrated on one or two platforms gets little from also maintaining a presence on three more where that audience barely shows up.
Add a platform only when the first ones are stable
A reasonable approach is establishing a real, consistent presence on one or two platforms before adding another, rather than launching everywhere at once and spreading a limited amount of time thin from day one. A platform added once there is capacity to sustain it starts stronger than one added prematurely.