To be perfectly clear, an Owner Number is something that a company assigns to a particular person who owns the minerals. In short, it is an account number. This is not anything issued by any governmental agency. While it may be helpful for the company to know the account number of the last payee, it does not excuse them from payment if you don’t have it. The royalties and the obligation to pay them don’t go away.
Rachel is correct in that a probate will probably be needed. If more than one estate requires a probate they can usually be combined into a joint probate to save time and money.
Also the estates may qualify for a summary or ancillary probate procedure which saves time and money.