You ask about finding your father’s API number. API numbers, which stands for American Petroleum Institute, are indentification numbers assigned to wells so there won’t be one for your father. The check you got from Legacy may show an Owner Number but that will only apply within their accounting system. The “R” type interest you mentioned probably refers to a royalty interest. The Shelby County Appraisal District may also have a tax account number for your grandfather’s producing mineral interest.
According to the Railroad Commission, what was originally known as the Doyle Boles Federal Lease is a 674.359 acre pooled unit, and it includes three wells, all producing.
For some reason the name of the Doyle Boles Federal was recently changed to the USA Cothrom Unit. That was done about the same time Legacy changed their name to Revenir, but I don’t see how that would be related.
The original well in the Doyle Boles Federal that was drilled by Marathon in 2010 is called the K 1 H and is API 419-31564. Legacy took over as the operator in 2015.
Legacy completed two more wells in that same 674 acre unit in 2022. One of those is now called the USA Cothrom A12 (API 419-31827), and the other the USA Conthrom A14 (API 419-31828).
On your question about how to find out how many mineral interests your father owns, it may require some research in the Shelby County deed records. If your father inherited mineral interests that were originally owned by his father you need to determine if your grandfather’s estate was probated and if deeds were recorded transferring mineral interests from his estate to your father.
Here’s a link to the County Clerk of Shelby County. http://cc.co.shelby.tx.us/
A note on at site says while their system is being transferred to the internet searchs of their indexes can only be done in the Clerk’s office in Center, Texas, but when you click the above link on the first page that opens click the heading marked Real Estate and it looks like you may be able to do name searches in the deed records. Try checking for things that have been recorded in your grandfather or your father’s name. You may need to do the same thing in other counties where you think that your grandfather might have owned minerals.