A customer ID is the ten digits Google uses to name one advertising account — written 123-456-7890 in the Google Ads screens, and 1234567890 everywhere an API takes it. Every call here names one: what you report on, what you propose a change in, what the record of changes belongs to.
An account is given, never found
You do not search for an account and you cannot name one you were not handed. GET /google-ads/v1/accounts lists the ones your organization may work in, and every other call takes one of those values. An ID that is not on that list is answered as an account that does not exist — 404, never 403, and the same words either way, so nobody can tell a real account they may not touch from a number nobody owns.
That list also says what may happen in each. may_change true means a change can be proposed there; false means the account is yours to read and report on and nothing more, which is where plenty of live accounts start and stay.
Ten digits with or without the dashes, either way. The API normalises what you send, and answers in the dashed form a person reads.
Questions people ask
How do I get an account onto that list?
Somebody gives it to you. Either a partner assigns one of its linked accounts to your organization, or your organization names an account of its own and asks for specialists to look after it — and in both cases an invitation is accepted inside Google Ads by whoever owns the account.
Can an agent guess an ID?
It can send any ten digits it likes and will be told the account does not exist, which is the same answer it gets for a number nobody has ever used. There is nothing to learn from trying.
One account or several?
As many as were given to you. Each call names one, so an assistant working across several is simply naming a different one each time.