DMARC is layered across (/is useless without) two other protocols (SPF, DKIM), both of which also require DNS records.. there's no way to DMARC with one record.. unless you're excluding those SPF and DKIM records in your count. You /could/ DMARC with two records: DMARC+SPF, or DMARK+DKIM but it's ill-advised - use all three. (Ignoring host hierarchy/cname delegation, you can have far more records)

I suppose I'm splitting hairs.

Yes, a proper email authentication stack has DMARC + SPF + DKIM. But each of these is a separate protocol.

This also doesn't take into consideration the other protocols that have arisen BIMI, APRF, MTA-STS / TLS-RPT and are at even lower levels of adoption.

MTA-STS is useful (but it's about helping receivers by demanding mail is sent encrypted, not senders with delivery).

BIMI is a pay-to-spam protocol that helps senders, you can add a rule to postfix to mark it as such.

APRF is a draft, and likewise helps senders, not receivers.. not sure why anyone would want to adopt this standard. A little closer to DMARC reports, but sharing PI that you owe no sending server.

Were you previously sending email without either DKIM or SPF?

Because i doubt that has worked at all in the last decade.

Are you replying to the wrong comment? No on all counts.

Then setting up DMARC is only one record because you already had the other records.

Do they literally just need the DMARC record to exist though, or do they need it to actually say something? Do you need DKIM? I found that SPF is very easy but DKIM is a royal pain.