I asked Plaid and that's what I was told. It's probably larger than that now since they're constantly adding long-tail support.
Do you have to be a paying customer to pry that information (i.e. list of banks they support) out of them?
Besides, the account is limited at the moment. I get the following error:
{
"code": 1306,
"message": "invalid institution permissions",
"resolve": "Your Client ID does not have access to this institution endpoint"
}