A USCIS online account number is usually a number offered to a person who has filed a certain application through the online filing system of the USCIS. It is also possible to receive this account number in case you’ve filed a paper through the USCIS lockbox facility. Used to be called USCIS ELIS Account number.
Majority of immigrants don't have it. Going on your USCIS account settings will not reveal your number, which is why it can be a little confusing trying to find it. But if you receive a notice of action letter, you should look over it carefully and you should be able to find your online account number this way.
There's no way of obtaining a USCIS online account number. Creating USCIS account won't help as it doesn't assign you a number. The good news is that it's not required for the majority of applications with USCIS, so just forget about it.