I would like input on the following situation:  Taxpayer owns an LLC - he thought it was a single member LLC.  Spouse handled all financial matters and had tax preparer file returns showing a Partnership with son for 2019 and 2020.  Then 2021 was filed as 50/50 Partnership with spouse.  During 2022 couple filed for divorce.  Taxpayer is adamant that the business is a single member LLC and wants to file a Schedule C. 

At this point, taxpayer hasn't been able to obtain copies of returns prior to 2019 so we don't know how the entity was treated prior to 2019.  The originating document establishing the LLC (as filed with Secretary of State) is silent as to number of members.

What documentation should be used to prove the number of members and therefore the form to be filed for 2022?

Thank you for any input on this situation.